For normal range of motion in a squat, you have to have enough motion in the hip, knee, and ankle. Even if they have this, however, they still have to be able to maintain it while they oppose gravity. Sometimes giving them a load to hold in the front, an anterior load, will allow them to sit back in the bottom of the squat.
There are times when the shape of the subtalar joint in the ankle just doesn’t allow for enough dorsiflexion to squat. They may need to keep their heels elevated if a deep squat is required. Maybe they would get a lot out of weightlifting shoes.
If the squat can get pretty, but won’t stay pretty in all situations, there’s usually a problem with managing gravity. You need to teach them a new way to manage this gravity.
You could even do it this way:
1.) Four weeks with a plate squat
2.) Four weeks with a goblet squat
3.) Four weeks with a double kettlebell front squat
4.) Then you try the barbell
And sometimes it still won’t work.