Did you treat all of the birds at the same time, and did you follow the dosing instructions on the pack?
The problem with sulfadimethoxine is that it is an antibiotic, which you don't want to keep giving to all your birds unnecessarily. The best thing to do would be to get a sample of the poop that is worrying you and ask a local vet to check it for coccidiosis. Even a vet that doesn't normally treat chickens should be able to do that, as puppies and kittens often get cocci as well.
I recently had a very similar situation to you - one two-month old chick that I thought hadn't got over the cocci after treating with sulfadimethoxine, even though all the others seemed to be fine. When the vet checked her poop (and he didn't even charge me for it) he said he couldn't find cocci, but he did see worm eggs, so I wormed them all and she's been fine since!
The problem with sulfadimethoxine is that it is an antibiotic, which you don't want to keep giving to all your birds unnecessarily. The best thing to do would be to get a sample of the poop that is worrying you and ask a local vet to check it for coccidiosis. Even a vet that doesn't normally treat chickens should be able to do that, as puppies and kittens often get cocci as well.
I recently had a very similar situation to you - one two-month old chick that I thought hadn't got over the cocci after treating with sulfadimethoxine, even though all the others seemed to be fine. When the vet checked her poop (and he didn't even charge me for it) he said he couldn't find cocci, but he did see worm eggs, so I wormed them all and she's been fine since!
making it my top priority to get a sample and into the vet.