If you bought the Frontline from a Veterinary clinic, it comes with a guarantee. Call the vet or the number on the product. They can help you out.
Also, take the collars off of the dogs when you put the stuff on. Often times it soaks into the collar instead of the dogs skin. Make sure you are using the correct dosage. If you dog is close to the weight cut off, pick the larger dose. Vacuum, vacuum, vacuum, put a flea collar in your vacuum bag to kill the little buggers or toss the bag after vacuuming. You may want to treat the environment too. I you buy that at the vet it will also be guaranteed. These are a few tricks I learned before kids when I was a certified vet tech.