No fleas?

Hello everyone, I put Frontline on my cat around noon on Wednesday. There as
been a good improvement, but I still him scratching from time to time
(although no way like he was doing before). He scratches and nibbles with
his teeth where he still get flea bites. I still find a handful of fleas. I
have heard how great Frontline is but should Jacob still be scratching after
3 days from receiving an application of Frontline?

Thanks,
David Lee