As ninjouzata said it may just be their environment and tank mates do not make them feel secure - american cichlids often benefit from what are known as dither fish, these are fish smaller than the cichlid generally schoolers but livebearers will work also that are always active and show the cichlids there are no predators around. An empty tank can often make cichlids nervous and as a result show nervous behavior and hide a lot.
Wills