That sounds like a snake is eating the eggs, they don't leave any evidence. I had a five feet long black snake eat four eggs out from under a broody hen then disappear for three days while it digested them, then came back for more.I see no evidence of egg shells anywhere from the 8 missing eggs.
This could also suggest a snake is eating some of the eggs.The other hens have dropped in egg production because they have been stressed at having to find a new place to lay their eggs.