My birds eat Flock raiser all year, with oyster shell on the side. they eat lots of other stuff while free ranging, so less of their base diet, but it's always out there for them!
Modern birds need a balanced ration all year!!! Producing 200+ eggs each year is a huge metabolic effort for the...
Also, make sure that it's not someone else; look for signs of other predators.
You might have a chicken problem, as above, and adding a better diet, and finding a culprit is best. Or, you could have a daytime outside varmit, like a rat or ?? something.
Mary