A brooder should be set up so the chicks can regulate their own temps by moving toward/away from the heat source. If the cold side of the brooder is too warm, it can cause just as many problems as if the warm side was too cold, so you may want to check and see if you have a proper temperature gradient going on.
If it's possible to set up an individual brooder for the sick one, that would be a good idea. You would have a better chance at monitoring and providing individual treatment. If you can't, then I would put sheets of white paper towels over your bedding so you can have a look at poop consistency. It's difficult to do this when it mixes into the bedding and the paper towels will still give everyone enough traction.
The booster you are giving seems to be decent. Make sure the sick one is actually consuming it. If you think it might be dehydrated( which is possible if it is too warm or too weak to drink enough) then provide an electrolyte solution via dropper or small syringe. You can get packets from your feed store (best), or in a pinch get some unflavored Pedialyte (some people use Gatorade, but Pedialyte is better).
Something else to add would be additional PREbiotics. A lot of chick starters already have some, but adding some in the form of finely diced raw garlic or wheat bran would help those beneficial bacteria found in your probiotic chick booster set up shop more easily in the chick's gut. If you do the garlic route, be sure to offer chick grit as well to aid in grinding.
This is all I can think of without actually having your brooder and chick in front of me for hands on. Good luck!