If you live in a very northern latitude with very dark days, if your chickens are cooped up most of the time and don't see the sun, they will have poor calcium absorption. Or if she may have been eating things that prevented calcium absorption like spinach does, it robs the hen's body of calcium.
Poor quality oyster shell prevents proper absorption if it is mostly small powdery particles. Check your supply and replace it if this has been what your hens have been getting. Larger particles are absorbed far better than dust. Making it free choice so each hen can eat as much or as little as she needs is better than putting it in the food you give them.
If you are not feeding a commercial chicken feed, perhaps they don't get enough of added vitamin D and other minerals necessary for calcium absorption.
Or this may have been caused by a genetic flaw in her digestive system that prevented proper calcium uptake. We can't know, only guess at the possibilities since we know nothing about your situation from the small amount of information.
