In reply to mark s:
Fleas were most often seen at this time of year in the UK in a survey by the RVC a few years ago. Increased pet time in the house, central heating and the fact that most people consider fleas to be a summer seasonal problem causes this. If you are seeing a few fleas on your cats, there are thousands in your house. Cat fleas only spend their adult life on the pet and don't move animal to animal (rabbit fleas are much more clever) so eggs, larvae and pupae are all in the carpet, crevices and other favourable sites around the house.
It will take a while to kill fleas which have pupated as they will quite resistant to chemical attack. Repeat treatment is needed.
A common approach would be
Maintain flea treatments for ALL the animals in the house and treat the environment aggressively
1. Hoover the whole house
2. Immediately spray with a product containing an adulticide and an insect growth regulator (such as Indorex already mentioned)
3. Leave the house empty for a few hours and then re-hoover
Sprays are bad for some other pets such as birds and fish and people with respiratory so READ the label
** Repeat after 8 weeks **
Why are you seeing fleas when you are using an adulticide?
Common reasons are...
1. You are using a product at the wrong frequency or in the wrong way
2. The environmental burden means that even though fleas are killed by the product they are replaced by new ones
3. The product has been washed off (only true for some)
Hope this helps a little. They are bad critters! A vet can help you understand why your regime has broken down and its worth a chat, particularly if the cats have any skin lesions as they may be able to be given some more immediate relief.
HTH.
Post edited at 18:44