Comp issues

I’ve had 3 comps for the past month or so. They are around 1.5 inches each, and went into a 75 gallon tank first. During their first week they were fine, swimming around and eating food with gusto. After their first week seeing that they had acclimated quite well I decided to add 5 Occies with them, the shellies were pretty small with the biggest only being slightly over an inch. The shell bed is located at the front right side side of the aquarium, leaving the back and the left side without shells. The shellies pretty much stay where the shells are. After a while though I have noticed that the comps have become reclusive, they no longer come out to eat, and are now always hidden in the rockwork. Could the shellies have caused this? I don’t see any aggression from the shellies, they pretty much mind their own business. I’m stumped, any suggestions? :-?