Allergies, while rare, are not unheard of.
Mites need a few treatments before they'll die off. I'd finish treatment before looking for other options.
I was going to say that Revolution may not be doing the job like Ivermectin would, but I wasn't sure about it. I just checked GuineaLynx and if I'm interpreting what they say correctly, Revolution has a hit and miss track record. Is it possible to ask your vet tech to give you Ivermectin instead?
http://www.guinealynx.info/antiparasitics.html Selamectin -- (Revolution)
A newer treatment for mites and lice requiring a prescription. Apply topically to the skin. Pfizer reportedly advises 10mg/kg for cavies.
Selamectin does not have ivermectin's proven safety track record but some cavy owners appreciate the convenience of once a month topical dosing.