My TAG came to me in a similar situation. We've been working with her for a couple years now, and the progress is slow, but steady. She had severe muscle atrophy from lack of use, and wouldn't even open her wings when bathing or to flap (she would flap to break a fall, so she had the instincts there somewhere). She wouldn't flap when raising and lowering her, so it was really hard to get her to do ANYTHING with her wings. If she was presented with a gap of more than 2 or 3 inches - something she would have to jump/flap to cross, she wouldn't do it. As far as she was concerned, she was trapped. (Per the vet, there was no physical cause beyond muscle atrophy from misuse, so we started working on the problem).
What helped us was playing with the position of her food bowl. We put two plastic bins next to her cage so that she had to cross the 1st bin to reach the 2nd. We put her food on a playstand on the 2nd bin (we left her water bowl in her cage, so she had to move between them). She quickly got used to walking across the bins to eat, so we very slowly started pulling the bins apart. I'm talking less than an inch to start. Each time she got confident with the adjusted gap, we widened it a little bit. It took about three months to get her to actually jump/flap for the first time. In the early days, I would also manually extend her wings and gently massage the muscles (what little muscle there was).
It's been two years now, and she still has a long way to go, but she has improved SO much. She can fly about 5 feet in a straight line and land on a flat surface or on the vertical side of her cage, and we're currently working with a setup designed to force her to ascend, which requires more strength. I suspect the issue there is not just one of muscle, though, but of not being able to see the landing spot before taking off. She hasn't worked out turning yet, and descending still makes her panicky, as she treats it as breaking a fall rather than a controlled movement, but you can tell her wings are slowly getting stronger and her confidence has grown a lot. I have future plans to teach her to land on a perch or branch (as opposed to a wide surface) and am pondering how to set things up so she'll be forced to start making turns in flight.
She recently started (finally) to flap when lifted/lowered on your hand, and also will flap on command for treats, so we can finally get her to do proper wing exercises beyond her short flights. She uses her wings in a more natural manner, little stuff you don't notice until it changes, such as flapping to get that last boost when climbing, and just stretching her wings out for no reason than stretching feels good. She spreads her wings to bathe now. She has worked out that she should crouch before taking off, and she leans forward and tucks her feet up in flight now (originally, she would fly almost fully upright with her feet splayed out ahead of her in panicky preparation for landing).
She's 13 and old to be learning, but she could easily live another 50 years, so there will be a lot of time to reap the benefits of the lessons we're teaching now.