Sometimes the fleas in your area build up a resistance to certain products and you have to switch brands. I used Frontline for years, but had to switch to K9 Advantix last year when I had a big problem with fleas.
Where did you buy your K9 Advantix? There are a lot of counterfeit products on the market. It's not safe to buy them at the retail level, only through your vet or online pharmacies like KV Vet, Dr. Fosters & Smith, etc.
A member on another forum got counterfeit flea treatment from Petedge last year!
Here is the EPA article which shows you how to check to see if you got the real thing.
EPA: Pesticides - Counterfeit Pesticide Products for Dogs and Cats