A: Your student may have an os trigonum problem, where a tiny chip of bone from the talus is getting caught in the joint.
Initially, there isn't any discomfort or pain involved. They simply can't point one foot as well as the other. The discomfort and/or pain typically comes when they begin to force and push their pointe, creating swelling and irritation at the back of the ankle. The os trigonum is diagnosed through an X-ray or an MRI, and I have seen a handful of very successful surgeries to remove the os trigonum. The dancers who had the surgery were in rehabilitation within days.
I also suggest you check her alignment and make sure that she is placed accurately on her feet in standing and at the barre. It's possible that being out of alignment, such as a big pelvic/torso shift, may be influencing the musculature of the lower leg.