What is her laying history? Has she been laying good eggs for a while and this just happened or is she just starting to lay. Are the others laying? How are their eggs? When I have a problem like this I like to decide if it is a flockwide problem or an individual chicken problem. I don't want to mess the others up if there is nothing wrong with them. If the other eggshells are fine then you have a problem hen.
I agree with Dobie, much better to give that specific hen a tablet as she said.
Chickens need calcium for general body maintenance, not just for shells for their eggs. All the calcium does not get digested from the eggshells or other calcium sources they eat. Some calcium goes straight through and out the back end. If their eggshells were the only calcium they are getting it is a losing proposition, both from body maintenance and out the rear end.
Chickens can get calcium from a lot of different sources. From their feed, some plants they eat, creepy crawlies if they can catch them, maybe even some of the rocks they use as grit if it is limestone. Maybe feeding them back their eggshells is enough extra with all the other calcium they are getting from other sources to where they do OK. Sometimes it is not necessary at all. Sometimes it is not enough.
Let your egg shells tell you how you are doing. If the other egg shells are fine then they are getting enough calcium from somewhere the way you are doing it so you don't need to change anything for the flock in general.
My preference is to offer oyster shell on the side and let them decide. If they need it the oyster shell will disappear. If they don't need it that oyster shell can last years. I offer it on the side, not mix it with the feed.