Susanna, there are many things that will stop laying. Mites are one, stopped vents are another. worms can do it, Marecks (sp) disease (there are 4 strains and they are only inoculated against one at hatching).
Most of the time you will find one or more egg eaters in your flock. To prevent them, get a roll away nest box. I prefer best nest box but there are other brands. then you may see an increase.
My ladies had mites and Marecks this last fall and I am still getting them healthy. Never had that happen in 50 years of raising chickens. A clean run and coop is important.
For mites use crushed chilies in their food. you don't need pesticides. I did that and it didn't work. My daughter in law who is Shoshone told me about chilies in the food and after 2 weeks they are better, for worms a little apple cider vinegar and some crushed garlic in the water fixes that. for Marecks use nano silver in the water. (not colloidal silver)
Other than that make sure they have clean water, greens and bugs and they should be healthy.