Mine did that too. No idea why, probably just didn't get the hormone change to do it yet. Last year I had one who went late & molted in February where everyone else molted in Oct - Nov. So she might just be a late molter. It sounds more like she's molting but slower and longer than the others. I have a cymani-orpington who has been slow molting all winter. She's laid at least 1 egg per week where everyone else had stopped laying when they molted & haven't restarted since the fall. Under her tail is noticeably bare, but she looks normal otherwise. Maybe an increase in protein will help your hen get her feathers filled in faster. I typically mix feather fixer pellets w/ layer pellets so they get a big more protein while still getting the calcium to lay eggs.
Good luck!