When I have something like this I try to figure out of it is an individual chicken problem or a flock wide problem. The rest are doing fine so it's obviously a single hen problem. That clearly shows you are doing nothing wrong. I would not treat the entire flock for her individual problem.
She's just a pullet starting to lay. The internal egg making factory is pretty complicated, a lot of things have to go right to get a good egg. It's not unusual for a pullet to take a bit to work out all the bugs in that system. It's possible she'll work it out in another few days. You might give her another week and see what happens.
It is possible she has a a problem. By instinct she should know she needs the calcium but maybe she doesn't have that instinct. Her body may not process the calcium she does eat. She may have a defective shell gland that just doesn't work right. These are the types of things that may be corrected shortly or may never be.
You can try feeding her separately to get more calcium in her. Maybe crush a Tums or take some powder from the oyster shell and mix that with a treat. Or follow Azygous's suggestion. It's possible she will straighten up or you may have a defective hen. Those do happen. But don't give up too quickly, patience can be your friend.
