I also have had some young ones occasionally do a mini molt or very soft molt. The range of normal is pretty wide. Seems like everyone has birds at one time or another that don't read the manuals! If everything else seems normal, I would probably not worry too much. The only other thing I can think of is, since you mentioned head and neck feathers particularly, is if there is any fencing or anything else that they might be sticking their heads through, which might have caused some feather loss. It does sound like molt from your description, but just wanted to put that out there.