Dog still scratching and digging after frontline?

I have three inside dogs. A boxer, a beagle, and an irish setter. The boxer and the beagle are fine. The irish setting is still digging and scratching after I've put a new dose of frontline on . I put the new one on about three days ago and I try and keep it fairly regular on the "every 30 days" thing. She has even broken out with a red rash on her stomach and back legs. Should I give her more frontline? I havent seen any fleas in the house or on the dogs this year. Tips?