Lots of reasons for this to occur. Like previous post, tank "could" be too new and not enough nutrients.
Could also be that you have not done a water change in awhile or balanced the params so could be out of whack.
Could be lighting, too much will cause them to shrink up.
could be a nuisance tenant like a fish or crab or flat worms etc, so a good dip could get them to recover and release pests.
torches really like random low to medium flow overall and they like a high nutrient system. We tend to keep our torches low in our water column to get less light and we keep nutrients high and skim but not overly skim.
The will bounce back if you get get a handle on it unless you start to see tissue receding meaning the flesh getting closer to the bony structure, then you may have issues with throwing a head or melting.
