I am not real familiar with Sentinel (I think the fleas only die when they
bite the dog, so you could have plenty of fleas around that have not bit them yet (?) Advantage works great on my girls...flea baths are never necessary. Maybe the fleas got into your house (rugs, etc) and you are having a population explosion? Years ago, a friend of mine was not aware of fleas in the house, but laid out white sheets on the floor...
then she saw tem jumping on the sheets!!
I guess she had to treat the house then after that. Good luck!