i have porcelain crabs (my favorite crabs). when i first got mine, they were hard to spot. they never come out in the open. they may be feeding in the rear of the tank behind the rocks. mine do this occasionally. they also like to hide in crevices. they are very hard to spot until you know where they make their home. try taking a flashlight and looking in the rock crevices. i got mine in june from this site.
if you don't have a background on your tank, check behind the rocks. my peppermints like to hang back there also.