How long does it take Advantage 2 to kill fleas on my dog?

I treated my dog with Advantage 2 four days ago and last night I found a live flea on her! Advantage 2 says it kills fleas, larva, eggs and ticks. Is it normal to still find one alive after three days? I also treated the carpets and upholstery with Borax and salt about two days ago. I don't know why these darn things wont go away! Should I be patient or am I doing something wrong?