Frontline, but still some fleas?

I put Frontline on Jack and Asher on Sept. 9. It's only been 10 days, but I noticed about 3 or 4 fleas still alive on Jack. Is this normal for some fleas to still be alive? How long does it take for them to die off? I got six months worth of Frontline from the vet when I took Jack and Asher in.

No, the fleas should be gone pretty much right away.
I'd call the vet and report that.