I've had a rash of soft shell eggs this summer from not only my new layers, but my 1+ year old girls as well. Before we got the new pullets the older bunch had been on layer pellets, but still with oyster shell on the side. They never had soft shell issues until about 2 months of being on flock maker crumbles. Anyway, I ended up buying some of the flaked oyster shell vs the crumbled. They have been eating that better and our soft eggs issue seems to have resolved 

. Also, I sprinkled some around on the ground as well as putting it in the same dish and finally saw them eating it. So what I have learned at least from my flock is....
Flaked is greater than crumbled oyster shell, and oyster shell on the ground is superior to it in a dish 

 dang chickens.