[ladyjane]

About Sentinel:


Sentinel Frequently Asked Questions - Dog Products - Novartis Animal Health Canada

From that site:

I saw an adult flea on my dog after I gave him Sentinel, does this mean it's not working?
Sentinel does not kill adult fleas, so you may not immediately see how effective it is, but rest assured that adults currently on your pet will die off without reproducing. If you wish to take a more aggressive approach to killing the adults Novartis recommends treating your pet with an adulticide for the first month or so.