Have you tried using a flea comb? It is a very fine tooth comb, usually metal, that with catch any fleas when you brush your kitty with it. Just get a bowl of water with a few drops of liquid soap in it, stir the soap around with your finger (you don't want bubbles). Then grab a kitty and start combing. Take a few strokes and then check the comb for fleas. Any fleas that you do find, quickly put them in the watery soap solution. The fleas will sink and drown. Good luck, fighting fleas is hard but you will win with persistence!
It takes a while for the frontline to work. I use a different brand, but it states on the website that the animal must be bit by the flea, then the flea dies after biting, but the pet does get bit and therefore continues to scratch, but that does not account for all the fleas that still might be living in your house or in your garden. Fleas hide in many areas, blankets, pillows, sheets, comforters, fabric couches and chairs, matresses and clothing and eggs and larva get hidden in dusty areas on the floor in the cracks and crevices..The eggs and larva will incubate, stay hidden and alive in the dust and when they notice an animal or even get stirred up by vacuming or dusting, they come out of those dust bunnies alive and ready for mealtime.
I would suggest vacuming every day or every other day, vacuum under the chairs and footstools when it is darker and fleas love to hide. It also helps to understand the fleas birth and death cycle so you know when its the most important to change sheets and blankets and towels and get that pile of clothes washed and put away instead of just storing it on the table or chair.
I myself can not use a flea bomb. You have to leave the house, along with all the animals and there is always the question for me about the cleanup after a bombing, meaning china, dishes, glasses, food, open containers of food, and especially what is more harmful, the bombing of my living space or taking an active part in the removal of the fleas in the house.
If a person is proactive and stays on top of cleaning and scrubbing floors, yes, even wood ones can harbor eggs between the cracks, it could take 3 or 6 months to eradicate the fleas. Meantime, your frontline is working and killing the live fleas and any new eggs or larva that would have been laid.
Good luck to you....I do wish you the very very best in this...
M