A few points:
1. As the nurse at my local vet's surgery said to me the other day "If you can invent something which creates a force field around pets to keep fleas off them you'll make a fortune". Her point was that ALL flea treatments only kill fleas that have landed on (and probably already feasted on) an animal. They DON'T prevent fleas which are in your carpets (or just in the grass outside) from jumping onto a cat or dog. So unless your carpet and furnishings are 100% clear of fleas (AND your garden and wherever you walk your dog are too!) you should always expect to see a few fleas on any animal treated with Frontline (or with anything else) around this time of year.
2. Like you, I wanted to give a repeat dose of flea treatment rather earlier than it said on the packaging but, because of the risk of an overdose, the nurse wouldn't even sanction applying it a single day early (which is actually exactly what I'd expected her to say).
3. If you're using Frontline, an Asda pharmacist recently told me that Ectoline (which Asda sells cheaper than Frontline) is exactly the same product as Frontline but simply with a different brand name, so you could save yourself some money at Asda.