This can be for a number of reasons. The main reasons would be if you have a really bad flea problem Advantix Flea and Tick Control Treatment is not working as strong at the end of the month as it does at the start, so although it is still working some it is not enough to stop the large amount of fleas. If this is the case you will want to re-apply Advantix Flea and Tick Control Treatment every 2 weeks while fleas are bad. Another reason is that it takes up to a year to stop the flea cycle on your pet. This means that treating your dog with Advantix Flea and Tick Control Treatment the full year instead of just the hot months when fleas are worse will make it more effective when flea season is on. Most people dont have an extreme case of fleas and wont have to worry about it lasting less than a month.