I'd check a few things for starters:
1) Stroke path
How far is the rod tip traveling at either end of the stroke? Going past the 10a / 2p positions could cause the line to drop too far below the median.
Does the rod pull the line circularly over the top or does it pull it linearly from front to back?
2) Easing the start-up speed
Start with a slow speed to ease the line into a horizontal plane gradually speeding up until the line becomes full of tension before you 'hit' it or else the whip-over effect will send the end of the line closer to the water. Every caster experiences a lowering of the end of the line from the casting lane, but by easing it on the 180 reversal the likelihood of a slap is greatly reduced.
3) Loop size
Check whether the loop somewhat open or fairly small. A larger loop will 'flip' open with a larger diameter, increasing the potential that it will turn over close to the water. This is related to (1)
Starting forward or back too early emphasizes the 'whip over' effect at the ends sometimes causing the fly to drive downward. Find the right moment to ease it into the opposite direction.
Imagine the line to be passing through a horizontal cylinder in the air. When you stop, the bottom of the loop forms. Direct the top half (which is created by the advance your rod makes toward that stop) the imaginary cylinder, or tilt the backcast upward just a bit if near the bank.
The two major faults I can invision without seeing your cast are (a) open stroke with the rod going too far on both ends and (b) too much of a whip-over effect when the line reverses direction from timing or power application.
Keep the rod movement somewhat abrupt, flicking only after the line gets tight horizontally in the air. Ease the beginnings of each cast (back & forth) to avoid a hard flip.
If you post a video we can provide a much more in-depth analysis. Look up an FFF certified instructor in your area or just stop into your local flyshop and ask a pro to eyeball your cast.
Good luck and keep with it!
IFFF Certified THCI @ 2005
Capeflyfisher Guide Service
Island Hopper, Guitarist, Incurable Dreamer
and Founder, Worldwide Flyfishing Forum