Revolution Plus and hair loss.. should I continue using?

I take care of semi-feral cats who live in my backyard. I started giving them Revolution Plus a few months ago. One of the cats has developed a bald patch at the application site since I gave him this month's dose. It's not bloody or infected, just bald, about the size of a nickel. Sometimes he scratches at it.

I'm worried about giving it to him again next month. Is this a sign of an allergy or something that might get worse if I continue to give it to him? Should I look for something else that's comparable? I need something that takes care of fleas/worms since they're outside cats and I'm in Texas, which is a big heartworm area.

Thanks for any advice.

Species: Cat/Age: 3 years/ Sex: Male/ Neutered: Yes/ Breed: Domestic Shorthair/ Body Weight: Unsure, but he was around 11 lbs two years ago when I had him TNR'd.