frontline spray no longer woks!!!

I breed Devon rex cats which are flea magnets. I sprayed them with prescription, expensive frontline spray only a few weeks ago and today noticed that they are once again running alive with fleas even though they are in a cattery and never go out. When I treated the cats I also treated the whole room with a household spray, and washed all their bedding on a boil wash. Don't waste your money. It no longer works.
My other moggie cats which go out into the fields all day, wear poundland flea collars as do the dogs and there are no fleas at all on them. However I cannot use the collars on my rex cats as it makes them throw up.