I have some experience with filefish. I started with a wild caught radial filefish from diver's den. And it behaved exactly like you are describing. Hanging at the bottom with it's nose touching the ground. I ended up getting it to eat pellets with the powerheads off once they hit the bottom and it noticed them. I got it eating this way for a few weeks, but it never seemed right and never swam around like a normal fish. And then, not unexpectedly, it died.
I tried again, but this time with a tank raised pair of whitespotted ones, and they were a completely different story. Voracious eaters of literally anything in front of them immediately. One ended up dying of a parasite, but that was months later. Other one is still healthy as an ox. They don't really ever hide. Always out in the open swimming around. It even sleeps in the open, usually with its mouth touching a branching coral.
I know I'm not really helping you at all, but maybe try live blackworms. Short of that, I would try to contact whoever you bought them from and tell them what's going on. But if it is anything like the radial I had, then it may not get better.
Don't give up on filefish though. They really are awesome fish if you get healthy ones. Sorry you are dealing with this, I was pretty bummed about my first one. The one I have now though adds such a unique shape and behavior to my tank.
Good luck. I really hope it works out.