I'd wait until you get to see some still/advance footage of your own cast, the answer may jump right out at you. Then if you still don't see it, consider sending it to your instructor, or any willing instructor to see what they see. I would be honored to be included in your list of reviewers.
I am by no means the authority on single-handed casting but I've been working my single hand casting a lot lately and have some thoughts to share. Others may chime in as they will likely have thoughts to share as well.
Smooth acceleration, straight path = correspondingly tight top leg of the loop:
The most informative descriptions I have come across lately are those that Bruce Richards has shared on the web and in text. I don't think you can go wrong by testing out the principles he describes on what movements relate to what results in overhead casting.
In general he states that the smoother and straighter the rod accelerates in it's path from backcast to stopping point, the smoother and straighter the top half of the loop will be. The more abruptly and the closer to this straight path the rod stops, the tighter the final loop will be.
Keep in mind that a convex path where the middle is humped upward will result in a wide weak loop; and a concave path where the middle in caved downward will result in a tailing loop / line collision. Straight is the only way to go, and I practice by thinking about the line from end to end in the air coming straight as opposed to where the rod tip is - it's easier for me to refine the line's vector than that speedy little tip on the end of a flexing rod. The end result is the same.
In a conversation at GGACC, Tim Rajeff once described line speed as the velocity that a well-formed top half of the loop reaches during flight. He used the word 'javelin' somewhere in that description, words I use to visualize the goal of a straight line path and smooth acceleration to this day.
Rate of change (acceleration), not speed:
It's not really about overall speed, it's more about acceleration - the rate of change in speed from start to finish. In fact anything other than a smooth ramp-up of acceleration works against you. The line will travel best with lots of momentum, and efficiency in it's flight path. You can add power later by increasing the rate of acceleration, and a little goes a long way.
Acceleration is misguided unless it is directed in a very straight path. For example, if the path is curved the cast will curve accordingly and energy will dissipate into thin air. If the path is headed straight ahead but the final rod stop pushes upward, a tailing loop will result.
I would recommend concentrating on low power, smooth acceleration first - then add more acceleration once the finesse is back.
Lengthen the path - but don't change the angle of the rod:
Aside from increased rate of acceleration from start to finish, the other trick to getting long casts is to lengthen the distance (path) that the rod tip travels from start to finish, hence making room for more acceleration, etc. This is how Lefty gets to the far end of the pool with just a little flick to finish a long smooth effortless stroke, a thing of beauty.
Although the belgian cast and long stroke methods work even when the rod is tilted almost horizontally, the most efficient method of creating a tight loop is to maintain the proper angle of the rod and it's loading properties regardless of how long you lengthen the stroke. In fact it seems that maintaining the proper angle of the rod lessens the amount you need to lengthen the stroke to achieve the same results, but I could be wrong. However that's the impression I get from experimentation.
Stopping - the bottom half of the loop:
The cast should end with an abrupt stop, which is different from a 'big wallop' stop which is quite common. Brian Niska of Whistler Flyfishing is a superb single-hand caster and refers to the stop as a sudden absence of acceleration rather than a hard stop, which is an interesting concept and should be tried.
When the path of acceleration is straight, and the speed of the rod has been ramping up pulling ever-faster against the other end throughout the path, all the rod has to do is stop pulling and the line will continue forward it it's path. In fact it's very useful to practice getting more of the power to manifest itself during the acceleration while putting less emphasis into the stop. This usually results in a super-smooth loop free of any turbulence, and if the rod stopped close enough (and just under) the top leg then it should be as tight as could be.
Although precision casting is often done with a vertical loop (top/bottom vertically aligned) most practical casting (e.g.clousers) is done with the top loop offset to the outside of the bottom loop slightly. 45 or 60 degrees off horizontal is common, safe and efficient. In fact some expert casters feel they have more power out to the side, though more accuracy close to the side. Your results may vary.
Finishing the cast: tucking and drifting:
Since the rod tip is deflected by the stroke during the cast, the recoil upon release of tension can create turbulence in the loop. The classic example is the tailing loop, which is also caused when the energy coming off the rod at the stop (bottom half) is pushing in a direction that is higher than the direction of the path of acceleration (top half).
It's often a good practice to slightly 'tuck' the rod tip under the path of acceleration (POA) to finish it, which creates a loop that is energized top and bottom. I play around with 'aiming' that tuck so that it's more parallel to the POA which negates some of the deflection in the bottom loop.
If done with too much gusto, or done too far forward/downward (past 10 o'clock) may put a large deflection into the rod causing turbulence to enter into the bottom half of the loop, pulling the top half out of the path of flight and weakening the cast.
Steve Rajeff talks about adding a final drift to finish the cast to dampen the recoil of the rod at the end of a long, powerful cast. This helps to reduce the turbulence that can be introduced into the bottom half of the loop when casting for distance, and might be something to think about in your example.
Other things to try:
You might go out in the yard and try an easy, smooth acceleration - which for me is the sensation that the rod is always moving a little quicker than the line and pulling it along at an increasing rate / tension. Then stop the rod abruptly as close to that path tucking the rod tip just under it with the final flick of the wrist, a little laser loop will emerge. Start with low power, high smoothness and don't worry about overall speed - concentrate on the rate of change over the distance from back to front.
While working on this, I found that even a moderate amount of acceleration in a very straight path and an abrupt, dampened stop close to this path will shoot the entire flyline or close to it with anything from a 6wt to 10wt. The amount of effort involved defied the results and made me wonder why I had been working so hard all these years.
Two factors directly influence distance casting - acceleration and length of path. Once you get things squeaky clean add more acceleration and a longer path - but keep the rod angle correct when lengthening the path.
a) clean backcast
c) straight path of line from rod tip to leader when coming forward...
b) smooth acceleration (vs. overall speed)
d) lengthening the path without dropping the rod tip
e) abrupt stop with minimal downward deflection
Just to be clear, I am no casting guru just a dedicated student. I am posting this in an effort to be helpful, but take it with a grain of salt. Others may rebut the things I have put forth, and debate is good - however I just wanted it to be clear that my intentions are just to share what others have shared with me on the topic per the question you asked. I sincerely hope it's helpful in some way!