Is there a lack of motion in trying to do this movement? Or does it feel like you don't have enough strength to hold you? You may have both issues to contend with. Most large ballet companies have a physical therapist on staff or associated with the company. This would be a good question to refer to that person.
A recent study on hamstring graft recovery after ACL repair may help you understand what's going on. It seems that when a piece of the hamstring is removed to graft in as a substitute ACL, the graft site grows back, but the full hamstring strength doesn't return. Researchers compared one leg to the other and measured strength in three
They found the greatest loss of knee flexion strength occurs just in the position you are having trouble with: full or deep knee flexion. It's most noticeable when lying prone (face down) while trying to bend the knee fully. Dancers and gymnasts report problems
with knee flexion while standing. Wrestlers have trouble with a hooking action used to pin an opponent.
You may be able to get this final motion with strength back. A specific rehab program will have to be devised based on your joint motion and muscle strength.