@prairie that's not unusual at all. If they were the same age especially. Some birds cut back laying when it's really hot. Some birds start in molt in July. Normally it isn't that early but this year was different. A molt will take about three months. About the time the molt gets over then winter and long days set in and they stop laying. The trick to beating this is to have birds that are hatched at different times of the year so their cycles aren't the same. Also having different breeds can help as well. You can stimulate laying by having lights on a timer come on really early in the morning then shutting off when it's plenty light out. I usually had mine go off about 9:00 just in case of those gloomy winter mornings. I try not to do that though because I feel the birds need a rest now and then.
I have some lights in my hen houses that I left on because I had moved an entire group out there and wanted them to find the houses when they weren't used to them. I didn't want them sitting outside in the cold since they didn't know where to go.
I still have three groups of birds that aren't laying and haven't for months. One is in an outside pen and the other two are in my building. I The ones in the building have the exact situation as the other 12 groups in there but they just haven't gotten started up again.
The other problem would be if you bought adult hens from someone. They may have not been totally truthful about their age and sold you old hens. This can be particularly true if they were sex link type hens which burn out in a couple years.
If I were you I'd be looking for pullets that aren't quite old enough to lay. Some hatched this fall would be perfect. They will start laying in a month or two and you won't have someone else's cast offs.