keep doing what you're doing - for about 2 weeks. Make sure you use one of the special combs that has 2 lines of teeth to catch the little critters in. One day you will come and there will be no nits....................til the next time........
My DC have had them once so far and DS1, in particular, had hundreds of them. It took the Nitty Gritty comb to get rid of them all eventually (there were some very stubborn ones!!).
That was in April and I went out and bought the Vosene shampoo and spray in conditioner - they only get their hair washed with the shampoo now and I use the spray to flatten their hair down (both my DC have 'wild' hair) in the mornings - neither have had them since (although I appreciate it has only been 3 months )
Make sure you put loads of conditioner on and leave it for at least 5 mins before combing. Lice breathe through holes in their shells, conditioner blocks them and after a few mins they fall asleep due to lack of oxygen. If the lice are awake they can move from uncombed hair onto combed hair but this way you can be sure you'll get them all.
I don't think you could do this with DD's, but when my DS's had nits, I got them straight down to the barber for a No1.(I told the barber they had nits and he was happy to shave them) No hair = no nits. Worked a treat.