I'm having this same problem with my hens at the moment, it really is loosing me so many eggs!! I've took the culprits out and hey too are in a 'sin bin' (!) but I think they have tought the others how to do i aswell!!!! I've tried putting mustard in the egg shells but I think they thought I was serving up a nice meal for them - they loved it!!!!!!!!!!!!!!!!!!!!!!!!!!! A my wits end over this - HELP!