Laying feed should have calcium in it so even if one of the hens isn't eating the oyster shell she should be getting it from the feed. One of your ladies might just be laying a thin-shelled egg and it breaking when laid of getting broken as they step into or out of the next box.
I read a post by someone the other day that gave a half of a calcium supplement (of some brand) to a hen that had been laying a soft/thin shelled egg. They put the half of tablet inside some peanut butter and the hen inhaled it. They said that the next day the hen laid a normal, solid-shelled egg. It didn't say how long that one tablet made a difference in the egg shells of that hen but it did caution not to give the supplement regularly in that it could be hard on the hen's kidneys.
Maybe you could sprinkle a little oyster shell in with their feed and see if that makes a difference.
And, it *could* be that one of the hens are intentionally breaking the egg...does it look like most of the yolk and whites are there? No egg on the beak of one of them?
Just some thoughts...
Best wishes,
Ed