Most birds over a year old will molt in late summer or fall. After they are done molting they often do not lay again until the days have more hours of light. I know some have successfully added artificial light, but I don't know the details on how to be successful with that.
This time of year most chickens over a year slow down on laying, but that doesn't explain your summer. What food are you feeding them, and is there a possibility they are laying where you don't find the eggs?