If you can't identify the culprit/culprits as above, look for eggy beaks, and suspect those which hang around the nest boxes, also consider other possibilities - rats, lizards, wild birds.
Do you have any laying soft shelled eggs? Common with older birds, especially commercial hybrids and also with birds commencing lay - those are the eggs most likely to be eaten, and those most likely to eat. Try putting several golf balls in each nest box (confusion factor) and make sure that there is plenty of nesting material, or use rollaways. Make sure that there is a constant supply of self serve shell grit for calcium.
If none of the above helps you will have to use a process of elimination and separate your flock into smaller groups. If you don't resolve things soon, the other chooks will follow suit and the problem will escalate