I had a lot of trouble with poorly formed egg shells with my first six hens. The BO had eggs break inside her. I wrote about it on websites below. She lived to be almost six but she had the problem for the last few years. I've learned they don't have to be sick, the part of their "egg maker" is just working correctly to produce a nice covering. Don't get discouraged, it doesn't sound like they are sick. Just make sure they have quality feed and oyster shells. Stress seems to cause rubber eggs in mine. My granddaughters were here this week and we got a "rubber" egg. Perhaps when you can let them out again, it will correct itself.