Have you tried the liquid which goes on the back of a cat's neck and lasts for a month - it's always worked well for me. You get it from your vet and the one I use is called Advantage.

My cat has a Program injection at the vets every 6 months. Works for him. If you already have fleas in the home you must treat the home as well as the cat. You can get a spray from the vet

Yup, spray the house as well and make sure your cat gets flea treatment from the vets. The over the counter stuff is useless.