For Corid (aka amprolium) to work, it needs to be in every water sourcec - no untreated water for any of the flock. And it needs to be given without vitamin supplements of any sort. If your hen gets poultry cell at the same time as you are giving her Corid, neither is working. Amprolium starves...
Once she's done with the Corid, dose her with a good poultry multi-vitamin. Corid kills the coccidia by depriving it of the B vitamins needed to survive. Unfortunately, your chickens also need B vitamins, so they suffer, too. I nearly lost a prize pair of porcelain d'uccles to coccidiosis. Corid...