That's kind of a trick question in that the most successful tactics I've used have been when the fish were hitting like kids in the candy store.
For those tougher times, well, I still haven't hita single tatic which I could bet money on. Some that have worked at least some times;
Go to the surface. Not necessarily a popper, a slider or crease fly can be better. There is something in a surface fleeing bait fish which acts as a trigger to bass and blues.
Fish outside the bulk of the blitz. Often there are fish cruising the outskirts picking off strays left behind by the main body. These may be less picky.
Try a faster retrieve. Again, that fleeing baitfish action can be a trigger.
Try a dead drift. Some fish may specialize in hitting sinkers, especially when frenzied bluefish are chopping large bait in half and pieces are sinking.
One thing often suggested that has rarely worked for me is going to larger baits. Don't know why, just never panned out.
I think the worse case scenario is when bait is so densley packed that fish are swallowing large mouthfuls with each pass. There's just no way to tempt them with a single fly. When I was younger, it used to frustrate me to the point of taking the fun out. Now, I just reel in my fly and stand and watch the show.