Have you taken your dog to any new places recently? Done some gardening? Your dog can pick up fleas from a walk down the road, or the beach or a cat or native animal may pass thru your yard, drop some flea eggs and then you could have a flea breeding ground if conditions are good. If the larval stage fleas (probably in your yard) feel any vibrations, (such as gardening/mowing) they can hatch out and away they go!! Just because you cant see anything in the bedding, it doesn't mean larvae or eggs arent there. Wash the bedding in a hot cycle in the washing machine. Vacume your carpets twice a week or more (in case they have laid eggs there - or between floor boards). A flea can still jump on your animal at any time - if you are seeing fleas, chances are they are very sick and the preventative is working. Do your dogs swim a lot? If they do, maybe you should stick with frontline as it is water fast.
Speak to Bayer (advantix maker) - they are very helpful. By the way, the tick prevention is only effective if applied every 2 weeks - same as frontline plus.
(With the changing of brands thing, I would suspect that the fleas would NOT build up a resistance within 2 months).
Just keep on with your prevention monthly (even thru winter). :D
