Keep the treatments up. If the fleas are under the house and can work up through baseboards then it might take awhile.
Fleas can lie dormant for over a year I have read. They are picked up by vibrations. They move toward the vibration and the pet gets infected. Then they drop off and etc. The one thing going for you is that every flea you kill is less to be around. You can spray with the flea chemicals, then lay carpet powder. It should kill off the infestation eventually.
There are two types of fleas also. Dog fleas and cat fleas. (This is what I was told.. I am not sure the scientific differences.) Dog fleas are fairly easy to get rid of, but cat fleas can infest dogs or cats. I am outside my actual knowledge of area so I may have it all wrong, it being second-hand information. Anyhow, those are what I have heard. If it is right, then targetting for the type of flea might be necessary.
Pets are always your friend no matter what.