If she is laying you can give her Layer feed. The problem may be that you have other pullets her age that are not laying and probably should not be eating Layer because of the extra calcium in it. I don't know how many other chickens you have or how you are feeding or managing them.
When a pullet first starts laying it sometimes takes a while for her to get all the bugs out of her internal egg making factory. They can lay a thin-shelled or no-shell egg, an egg with a really thick hard shell, a no-yolk egg, an only yolk egg with no whites, or a double yolked egg, let lone some other really weird eggs. Yours is only 15 weeks old which is really young. I don't know how you feed and manage your chickens but the problem could easily be that her body does not process the calcium she is getting rather than a lack of calcium.
A chicken can get calcium for some of the plants they eat if they get to eat plants, but they are unlikely to get enough calcium just from plants. They can get calcium from certain things they eat , like small critters with bones or some bugs with a hard shell though most bugs don't have calcium in their shells. Do you have access to bones you could crush for them? If you have access to oyster, clam, or crab shells or lobster, crab, or crawfish claws those are almost pure calcium. I'm not talking about the clear soft parts of them, those are not calcium, just the really hard parts. Some rocks contain calcium, limestone being the most common. You can feed egg shells to them. By themselves they can't get enough calcium from their egg shells but if they are getting some calcium from other sources egg shells might be enough extra to give them enough total calcium.
Good luck with this, it's not always easy to figure out.