they'll die within a few months. personally, i've never known anyone to keep them alive for longer than that. i couldn't tell you if it's a naturally short life span, or we don't know enough to keep them at this point. but, i think if one asks, "is it more likely than not __________?" we'd all agree it's us, not them.
anyway, to answer your question, scallops are little bookers, man, they're all over the place! it's perfectly normal. if you can target feed them something like phytos and diatoms, maybe others have other suggestions, i dunno, but that almost always to seem to be the problem. then again, once i heard, i never tried to keep one, either.