Don’t catch her.
As odd as that sounds, there’s a reason behind it. Noibat are able to teach themselves to fly by instinct. So long as their wing muscles are perfectly healthy, they’ll be able to figure out the whole flying process on their own in no time, even if they’ve had little to no exposure to their own kind.
However, to you, it will look like your noibat is falling off her perch. A lot. Rest assured, however, that’s exactly how noibat learn how to fly, and until they understand when they should start flapping their wings, they will simply fall, sometimes unceremoniously. It’s very likely that whenever you place her on her perch, she takes it as you giving her an opportunity to launch herself into the air, as it were.
Either that, or your noibat has gotten so used to being carried that she assumes you’ll catch her, at which point not catching her will also be a good thing, as it will establish that you expect her to fend for herself.
I would recommend placing a few cushions beneath her perch to ensure she doesn’t hurt herself and then setting her back up on her perch only when she lands. It may also be helpful to place a plate of her favorite berries on a shelf out of her reach to encourage her to fly. Eventually, she’ll get the idea and begin flapping her wings enough to avoid hitting the ground and to get at the plate of fruit you’ve left out.
On that note, before trying the above, it may be a good idea to take her to a pokémon center and ensure that her wing muscles aren’t damaged. If they are, then that’s the actual reason why your noibat can’t fly, and Nurse Joy will be able to explain what you need to do to care for her. Sometimes, such injuries are only temporary, and you may need to wait another month or two before your noibat can take flight. Occasionally, however, it may be a permanent issue.