First Shield Trio for Dogs?

Has anyone used First Shield Trio for Dogs for spot on flea, tick, treatment? Dispensed by the Vet. It is supposed to also kill larvae and adult fleas. I cannot find any info on the web but Banfield @ Petsmart is selling this now. We applied for the 1st time to our 2 dogs and it worked great for the 1st week or so, but they have fleas after the 2nd week. Should I reapply sooner than 1 month? Has anyone else tried it for more than a month?