So you can do a decent one on a wedge? In that case I agree you probably need to develop more momemtum going into it. In an ideal world you could do a back extension w/ no momentum, but if you need to compete it soon you might need to try this:
Try dropping into the squat, and at the last second 'shooting' your butt back. Your coach might not like this, and it might be a deduction now (wasn't when I competed L5) but it might help you get the hang of it.
Straight arm back pike rolls where you slam your arms like you would in a back extention, either on floor or on wedge, might help. You would be working the upper body without worrying about hitting handstand.
Could you get a double spot and have one person grab a leg when you try back extensions on floor? We did this in my team setting with two girls the same size as the spotted girl, and none of us are coaches, and we were able to do this.
Are you turning your hands in toward one another? Fingers should point in.
On tramp try back drop, flip over backward to stand, and then try shooting toes up to land in a handstand on tramp. Back drop shoot handstand, from a bounce to your back on tramp. Same action, just you use the tramp to bounce you to handstand.
Back to the no momentum thing. We do sets of sitting in a pike, roll back to candle stick. Do one to candlestick, right away one to candlestick but slam arms into floor a little harder so your body starts to back extension, then one right away thru to a pushup position. All from a pike seat. This builds the direct strength you use in a back extension. They might not be pretty to watch, but then you go back to normal back extension and it seems easy.
Finally you might try sitting on a trapezoid block and rolling back so that the edge of your neck is off the block (probably 2 feet off the ground block) and open armpits/shoot toes to land in a handstand on the floor. You would be steping out of the handstand and facing the block (the block is the long way when you're facing it from here). So you are sitting, rolling back and doing a back extension but your body is already lifted 2 feet off the ground, so all you have to do is land in the handstand on the floor, but its still the same armpit open action. You might want a soft mat to go onto or a spot because if you mistime it you could collapse on your head. But once you get the timing its great for working really straight arms and getting the feel.
Hope something here helped, I struggled w/ these in L5 quite a bit as I was already tall and it was a hard skill to get. Mine still isn't great like w/ perfectly straight arms, and back extensions helps clear hips on bars a lot so we work them a lot. They are a fun and useful skill though and worth getting. If you want to do progressions w/ giants and stuff back extension roll to half turn helps learn a blind change, etc.
