Cat vet here. Don't lose hope.
There are a few common misunderstandings out there about fleas and their treatment IME, even amongst veterinary staff. Here's my take, which I hope might be helpful.
First major point of flea control: flea treatments don't repel fleas.
This means you will still see fleas on the cats even after they've been treated - they've jumped on from the environment. This doesn't mean the flea treatment isn't working. The key thing is, the fleas that jump on won't survive to reproduce - they die. You are still addressing the flea problem.
Start thinking of your cats as walking flea killers.
By all means try a different flea product, but I guarantee you'll still find fleas on them for as long as the infestation in the house is at its peak. Personally I'd stick with Bravecto, it's a good one. Just apply it religiously every three months.
Second major point of flea control: flea pupae are resistant buggers.
Adult fleas are killed by jumping on your walking flea killers, the cats. They are also killed by Indorex.
Eggs are prevented from hatching by Indorex.
Larvae are prevented from developing by Indorex.
Pupae? Sadly resistant to pretty much everything. These are the strongest link in the flea life cycle, and the reason flea infestations seem to go on and on. They stick in the carpet and furnishings, you can't kill them with anything, and they can remain viable for months and months.
The fleas you are seeing on the cats (or yourself) are emerging from these ironclad pupae and jumping on board. They'll die of course, once the Bravecto gets hold of them. But we can't stop them emerging in the first place.
On the contrary - we want them to emerge from their cocoons. As pupae, they are invincible. As emerged fleas, they are vulnerable. We gotta get them out.
How to make them emerge? Mimic the movement of a host nearby - vibration. That's why vacuuming is so important. Do as much as you can. Also make sure your walking flea killers, the cats, have access to everywhere in the house.
Because the fleas can remain in their pupae for so long, it can take a long time to clear a flea infestation - but that's normal (unfortunately). Don't give up!
Here's my summary for flea control:
- Treat all pets religiously with a good flea product (Bravecto is fine). Don't be disheartened if you still see fleas on them. Those suckers won't last long once they're on the cats.
- Treat the entire house with Indorex (or RIP Fleas) using the Five Step Guide on the RIP Fleas website. You should only have to do this once or twice a year, so do it well (it is a faff, but it's worth doing properly). Their FAQ page has useful tips for use as well. www.ripfleas.co.uk/r-i-p-fleas-extra/
- Let the cats march everywhere through the house, killing fleas as they go.
- Vacuum daily. Pay special attention to areas where the cats sleep or groom, as there will be more eggs here. Remember vibrations are the only thing that can effectively reduce the number of pupae, which are the hardest life cycle stage to eliminate.
- Rinse and repeat. This is a war of attrition. You'll get there.
Good luck!