Do you have a mature and stable, cycled tank? Do you have an area with strong flow?
Just add small amounts of powdered food on a regular basis. Feeding other fish will also help circle food around the tank.
They need an area to hang on to in the strong flow like a driftwood or stone.
Also if there are aggressive / predatory fish they will want to hide.
They also hide for a couple days if they have recently molted.
Are their feeders blackish? Sometimes they are damaged by ammonia burn. Fresh, clean water is VERY important all the time. After a couple molts they should get better.
Check water parameters frequently and add water changes due to the additional feedings.