My chickens are NOT EGGspert layers!

What gives... 7 hens, no eggs since August. Three are 2-3 years old, the others are just under a year. They have been living together the whole time, no signs of a stressful social pecking order. They have been wormed twice in that time (no evidence of an infestation). No sign of mites. They have continuous access to 22% layer and oyster shells and clean water. The three older ones molted in October- November. All feather coats look healthy. No evidence of predators/ neighbor pet agitation, they are in a very secure coop/ run.

What else could be keeping them (even just 1) from laying?????

 

They are sex-links; I chose that breed for the "excellent" egg laying.... The older one produced very well up until that point. The coop/ run they are in has no other hiding places for eggs; they are not allowed out of the "compound"; The structure (coop, nest boxes and roof of the run) is treated periodically with Malathion as per manufacturer's instruction. The coop/ run can be seen in my avatar. I have a "dust bath" that is a removable box that is cleaned and refilled often; I even mix hardwood ashes with the new dirt when it is refilled. Mites/ lice stand very little chance; no evidence (including on the birds) exist.