Sounds like something is stressing them. Check for worms, mites and the like. Our six are still laying six a day. They were born in March too. Do you leave oyster shells out for them? They may not have enough calcium.
We feed ours Countryside Organics with a 10% supplement of ultakibble. Grit and oyster shells are free choice. They free range for an hour most days or we give them greens. We do give them treats, but not to the point of ruining their appetite for their feed.
Does seem odd you are not getting anywhere near the amount of eggs you should be, even at this time of year. The lack of production in the summer makes me think there is something stressing them from the get go. Would not be surprised if mites or some other parasitic thing wasn't going on.
If you keep on looking for the answer you will find out what is causing this. Don't give up.