Do you know it is just one hen with the problem, or are you just seeing occasional runny poops and assuming it is a single bird?
If the later is the case, look up cecal poop on this site. If it is a single bird for sure, get a fecal done and you will know if she has an unusual # of parasites that might cause diarrhea. If it is a single bird that is free ranging, she may be eating something different than the others (cat food, garden scraps, fruit ect) that can cause different feces. Confining her with only layer pellet will solve that problem.
Speaking of food- what are you feeding, and did you recently introduce them to a new area?