It sounds like you are feeding them right, maybe some free choice oyster shell wouldn't hurt though.
Do they have a run or yard they could be hiding eggs in? And I am guessing you haven't found any sticky, leftovers from eggs in their nest boxes right? Lots of times they will eat the shell and everything, the only way you might know for sure is if there is some kind of dampness or clumpy straw in the nest box afterward. Adding calcium and protien to the chicken's diet can help a lot if that is the case. I have noticed some older hens can pick up the habit for either the calcium in the eggs, or the protien.
If the shells are too thin and an egg breaks, the hens will always eat the egg, so it doesn't attract predators.
Could be a snake or some such critter is stealing eggs as well.
You have some nice looking chickens BTW! They certainly look healthy.
The red one, I am questioning her age. I can see the comb starting to grow a touch further back on her head and the bare spots in the neck kind of say "old chicken" to me. The skin around the eyes and wattles in the back being larger make me wonder as well.
Your other two, the black hens look young. I seriously doubt they are older than a year and a half, 2 years at the most. And again, they all look healthy and happy! I hope you get some eggs soon.