Cord and Sulmet help!
You may get differing opinions here. Sulmet treats the 2 worst strains of coccidia, usually the ones causing bloody stools. Corid treats all 9 or more strains. I would be tempted to just keep using the sulmet since you started it. If you have any that aren't responding to it, then switch that one over to Corid, which can be given 1-2 drops undiluted twice a day. Corid dosage is 2 tsp of the liquid Corid, or 1.5 tsp of the powder, per gallon for 5-7 days. Follow up afterward with some vitamins and probiotics or plain yogurt for several days. To make undiluted liquid Corid out of the powder: add 1.5 tsp of the powder to 2 tsp (10 ml) of water.
Sulmet can be hard on chicks, so it should be used exactly as on the label. It is a sulfa drug antibiotic, and they can be toxic to the kidneys and hearing if used incorrectly. Corid isn't an antibiotic, and is much safer to use. Any drug, though can meet with resistance, and if one is used without success, the other should be tried. Here are some good articles about cocci: