I'm surprised the advantage has stopped working, though it makes sense, after time there is a tendency for immunity to be built up. (Especially Frontline)
You could try Capstar, and continue to use Advantage as the preventative, unless you think it's completely stopped working for you.
Revolutions isn't a bad product either, though in America you need a prescription, whenever I get it I end up just getting it from Australia.
Also, be careful not to overdose products like that, a vet shouldn't have told you to just use more... :3