are you feeding them the write food? as they dont eat the same food as other fish. For example for my plecos i have to add algae tablets as well as the flake food for the other fish.
Mine will only eat, when the lights have gone off and if the food is actually in the flow of the filter so they can chase after it. I have 3 also and have had them for ages. They do not do very much when lights are on but when lights are off they are swimming about quite a bit looking for food.
I used to feed by crushing up flake under the water so that flow of filter pushed it past them, if the flakes stayed on top of water they did not eat.