I have had good luck with Bio-Spot also. You should be able to find it at Petsmart. If not try looking up Drs Foster and Smith online.
However, a vet I used to go to that I really trusted once told me that if you have indoor/outdoor dogs then you will always be fighting the flea & tick battle. He said that if a dogs was only indoor or only outdoor then you stood a good chance of at least fighting to a draw. In my limited experience since then I think he was on to something. You might try using a combination of oral and topical medicines if no single one seems to work. Good Luck.
P.S. Here in noth Georgia we have been having a problem with flies. Very bad so far this year.