Chicks need no heat after five weeks, especially in summer. If you are heating the garage, stop. Let the temp go down to normal ambient temperature. Too much heat can result in dehydration and heat stress.
Continue with five more days of Corid after this next week off. Start again next Sunday. Stop giving vitamins during the second round. But resume vitamins after the second round is complete.
For the weak chick, give it sugar water to drink with just a tiny pinch of salt in it. Gatoraid is okay if it's the kind where they left the sugar in. Sugarless Gatoraid is less helpful than the kind with sugar, even for humans. It's okay for all the chicks to drink it. Return this chick to the brooder. Sick chicks get well faster when left with their mates.
Just in case this chick is having problems pooping, and I'll just have to guess it may be, give it a teaspoon of coconut oil. If you use unrefined coconut oil, the chick may eat it on its own. If not, chill the oil and divide it into bite size pieces and slip one at a time into the beak. Get it all into the chick.
There isn't really anything in the information you've provided that points strongly in any single direction, so treating for coccidiosis, constipation, and heat stress, are about the best we can do. Let's hope it's enough to make this chick better.
Meanwhile, if you have been giving the chicks treats such as raw fruit or veggies or greens or seeds or corn, stop until you can provide the chicks with chick grit for proper digestion.
