C.B.
My GD has had head lice many times. I usually use Rid, but this time I got Nix. It is completely different. It worked the first time. It's really thick and you put it on and immediately comb through the hair and when you're done combing through, shampoo. The hair will stay "greasy" which is good because if there does happen to be one of the little boogers left alive, it can't attach any eggs to the hair shaft because of the "grease". That breaks the life cycle which is what you want.
After the last treatment and daily since then I have been putting Coconut hair conditioner in her hair. It's greasy. It will keep any lice eggs from attaching to her hair shaft so even if she is exposed again, she won't have a "break out" because once the live bugs die off, there will be no eggs to hatch.
Don't panic - this is very common and just takes some time and diligence to get rid of.