Hard to be certain from a set of still frames, but here's my best guess:
1) She's not hitting full extension on takeoff. The knees, ankles, and shoulders should hit full extension right in the moment she leaves the floor; of the shots you've shared, none show this exact moment so it's hard to be sure, but extrapolating from what I can see I would hazard a guess that if we got a shot of her right as she leaves the floor, her knees and hips would still be bent and her arms would not be fully extended over her head.
2) She doesn't have sufficient strength in the core and lower body. Now, I should clarify this one a bit: takeoff technique and strength can decently cover for each other's deficiencies. An extremely efficient takeoff can make up for shortcomings in strength, and a lot of strength can make up for shortcomings in takeoff technique; however, there is still a bare minimum level of strength required, and the less efficient the takeoff, the higher that minimum is.
But none of this can effectively be trained at home, imo, and if anything her coach is saying goes against anything I'm saying, it's safest to assume her coach is right and I am wrong