The only problems I have ever had with chickens eating the eggs is caused by one of two things. The birds where either out of water or an egg had broken and then they would eat it to clean it up. Its an instinct thing! And when that happens, they will only eat the broken one!
There are wives tails of how you shouldn't feed the shell back to the hens because that will cause egg eating..., well that is a bid bunch of BS!!! I always feed the egg shells back to the chickens. So NO, that doesn't cause it!!!
Janet, you had mentioned that your birds love the same nest box. I too have had that issue before and it usually ends up with eggs being broken. Another thought maybe is these birds are being picked on by some others and don't leave the next box too often.
I once had a little old Silky that lived an entire winter in a nest box. She guarded it with her life too! She wouldn't let any other hens into it! I guess she figured it was too cold to venture out of it as I only keep my coops around the +1 - +5 mark in the winter. I also had a very aggressive Orpington rooster at the time who would jump anything that moved!!! On second thought, maybe thats why she didn't leave the nest box?! LOL!
Anyways, have a look at what is going on in the coop and I'm sure you will figure out why the eggs are getting broken!