Are any hanging out in the nesting box being broody? It is Spring after all, the time they like to raise chicks and not lay for 3 weeks, sometimes much longer.
It could be the stress from the mites, it could be a molt situation, it could be their diets. Lots of variables. Has there been any major change to them lately? Any chance a predator is scaring them at night? Stress and molting are the 2 biggest factors I think.