Their eggs usually drop to every other day when they are 2+. Thats why commercial farms get rid of them at 18 months old because they don't lay as much. You will have the odd few who will lay daily for a bit longer. Maybes they just need a bit more time to kick in if their egg number is slowly...
Well if they are over 2 then that could be the reason. Its not that half of them aren't laying it will be that half are laying one day then the other half the next day. If they are just 1 and a half I would probably expect more. Are they up to date with worming and have you checked for lice etc...