You can always ring Frontline for more info
It is more than likely your dog gets reinfected by the cats next door. Try to keep them out of your yard and flea bomb your place inside and out. Repeat after 2 weeks I think (check the package).
To check or the Frontline is working, catch some fleas and put them in a jar. If they are effected by fleas they should die within 24 hours. I've been told they should be effected within 30 min being on the dog.
I know from clients that Advantage is sometimes more effective.