I was having the same problem. I don't know for sure, but I was assuming egg drop symdrome. The good news is, once they molt, their laying should return to normal.
It could also be that they just aren't eating their oyster shell like they should. I mixed shell directly in with my feed for awhile and that got them eating it again. Try doing that for a couple weeks.
Otherwise, it sounds like you are doing everything right. Maybe cut down on the "treats" so their % of feed is greater.
Hope this helps you.
Egg Drop Syndrome Incubation period 3 to 5 days Duration 4 to 10 weeks
Egg drop syndrome is an infectious disease of laying hens caused by a hemagglutinating adenovirus and characterized by thin shelled and shell less eggs in otherwise healthy birds. The natural hosts for EDS virus are ducks and geese, but has become a problem with chickens of all ages . The disease is most severe in broiler breeders and brown egg layer strains, less so in white egg breeds. EDS was first introduced into chickens through contaminated vaccine. Transmission occurs by any of the conventional methods of disease spread. Infected birds excrete the virus in the feces. Vertical transmission is considered the primary mode of spread. Clinical signs are loss of color in pigmented eggs, followed by thin shelled or shell less eggs. Egg production drops by 40 percent. Virus isolation should be done in duck or goose embryos or cell cultures of duck or goose origin. Harvested allantoic fluid or cell culture should be checked for hemagglutinating activity in chicken RBC . There is no successful treatment . The endemic form in breeders can be controlled by washing and sanitizing incubators and egg trays before reuse. In layers, molting will restore egg production. Prevention is through the control of vertical transmission. Endemic EDS is associated with the egg-packing stations, as contaminated egg trays can be a major factor in spread. Virus is also present in fecal material, so hygienic procedures are required.
