Stronghold take up to 24hrs to start killing fleas..
In this time the active drug (selamectin) is distributed systemically (blood stream) and topically (skin).
Don't worry just yet. It should still work. You may well see fleas after 48hrs, but these could be new fleas from the environment that haven't bitten the cat yet. You should start to see dead fleas in the cats coat. Newly hatched fleas in the environment wait until a heat source or close vibrations are detected. This indicates to them that a potential host/blood source is nearby. Regular vacuuming stimulates this response and helps speed up the flea life cycle.
Stronghold's ovicidal activity (ability to stop viable eggs) works for 3 weeks so it may be worth retreating in 3 weeks time)
Don't forget that if fleas are seen on a cat, then the environment needs treating. Especially cats bedding, carpets, cracks and under furniture because hatched flea larvae hide in dark places.