Becky gave some great info. I just wanted to put in my 2 cents. When I read your post, the first thing that popped in to my mind was dehydration. I have seen this 2 or 3 times in our colony, when I inadvertently leave a valve closed on the water reservoir after cleaning a bucket. This could also happen if they ran out of water, possibly from a flooded tub lower in the rack, or if one of your tubs has a clogged water nozzle. If it is your entire collection, and they are spaced out between two different racks, then the possibilities are less likely that you forgot to water two racks (I don't know your exact setup). As I said, this is what I thought when I first read the symptoms. Hope you can find a fix for your problem.