My first thought is mites/lice. Gapeworm is actually more rare than we anxious chook parents think. It's good that you treated for worms anyway though and doubly great that you used the right wormer for gapeworm! If possible I would recommend doing a follow up fecal exam at a vet's just to be sure. Some cat/dog vets will let you bring a sample in since it's the same procedure with chickens but just ask them ahead of time.
As far as mites/ lice go... Look at them during the daytime and also take a flashlight out to the coop after dark and check them AND the roosts. Sometimes the little bloodsuckers creep out after dark for a feast and scurry back to the crevices before daylight, leaving your chickens itchy and drained.
As far as DE goes, I know that some people swear by it but I personally would never use it to treat my chickens. It creates more problems than it actually solves. It can REALLY damage a chicken's respiratory system. I use it to prevent ants from getting into the cat & dog dishes, use it as quick roach death but never around the chooks and never where it would get airborne. For lice/ mite infestations I'd use Elector PSP which can be applied directly to laying hens and has no withdrawl period. The active ingredient is spinosad. I RARELY if ever use harsh chemicals, so I make these recommendations as an organic nut, lol! I do prevention naturally and intervention as gently but effectively as possible.
I'd figure out what's going on first though.
The other possibility is wet fowlpox. When you looked in their mouths, was there any sort of cheese-like substance or pimple-like bumps? Tis the season for mosquitoes galore, which can transmit fowlpox (amongst other nasties).
I forgot all about Wet Fowl Pox
