How old are yours? Most hens begin laying around six months of age, though some can start earlier and some can start later. Sometimes, if they turn six months old in late fall or winter, they will hold off until the spring to start laying too.
If your chickens are faily tame. The hens will often squat down when you walk up to them when their about to start laying. They may not do this when a roo is around. If you see them squatting in front of him they are about ready to start laying.
If you had a bird eating eggs you would find egg shells cracked plus the guilty bird would have dried egg yolk on their face. If you let them free range you may look under bushes or under a storage building or somewhere they could hide a nest. Winter may set in early this year and Cochins start laing later than most. It may be next Spring. They will be very good mothers.