I had the same problem with my girls at the beginning of our season! When I first formed my stunt groups, I placed the stronger flyer's with the weaker/newer base groups and placed my weaker flyer's with the stronger bases. I felt that this would balance things out, and for the most part it did. Except for libs! For about a month and a half they could hardly get the flyer to a half. I refused to switch the groups, just because my theory was working, for all of our other stunts

Eventually, one day I got in as the backspot because, personally, I can't fix a stunt until I really feel what's going on in it, and it was def the backspot's positioning that was throwing things off. Backspot needs to have one hand under thigh/bottom of the standing leg, in order to give a good push off, and backspot needs to have the other hand on the ankle of the free leg to really drive it all the way up. My backspots don't let go of the ankle until the flyer is stable. When the flyer is stable the backspot shifts that free hand to the standing leg, to help pull up the weight. If you're bases are weak and just don't have the upper body strength to get the stunt fully extended, get them on a conditioning schedule everyday. I have my girls do push ups, sit ups, etc. to build up their strength. Good Luck!