Substrate? If it's a bad substrate, could be a problem.
Tankmates? Aggressive tankmates can cause stress..
Lighting Schedule? Too much light or too bright of light could possibly stress your ray out..
Has the ray been losing weight even though it has been feeding regularly? (Could be improperly QT'ed before you got it, still has parasites..)
Water Changes, Heat(82-84), Salt, No Stress.. Feed Live foods like black worms, ghost shrimp to stimulate eating.. Try to get onto chopped worms or red worms.