Keep in mind that some horses can naturally carry themselves in a frame easier than others. Also, some horses tuck their head nicely in a "frame-like" position, but aren't truly using their backs, so aren't truly working through themselves. Chances are the horse that seems to "go in a frame easy" with 3 years off is just tricking you with her head.
Take your time, get the mare you're talking about in the OP fitted up - as Always pointed out, she's likely out of shape and simply cannot do some things.
Excellent points made so far - would like to point out that what works for one horse very often does not work for another! I would have a chat with the owner - most likely the horse needs some conditioning and, at the moment, just work on going forward. Could be that she needs a once-over by a vet as she may be sore.
For now I would just work on her going straight and forward, if you don't have straight and forward you don't have anything to work with. Especially if the horse is unfit, she's going to need to build up the correct muscles, she wont have them any more
Let's forget the word "frame." It's totally unclear. In another post you said you couldn't get her to track up. This is something I can understand. Is that the real problem? If it is, then you need to find a way to activate her hindquarters. If your legs aren't doing it, then you need to use a whip. Or maybe you're blocking her action with your seat.
You have an instructor, so why can't you ask your instructor where the problem is?