While there is much evidence that lice are becoming resistant to the effects of anti – lice shampoos, the most common reasons for treatments to not work is failure to remove all of the nits.
Another common reason is that your child is continuing to be exposed to someone with lice, either at school or daycare. Make sure that all childcare personnel know to do lice checks on all children to prevent reinfestation.
After treatment, if you continue to see nits, but no live lice, then that may not really be a treatment failure. Nits, even when empty, can stay attached to the hair for a long time. Continue checking your child’s hair and scalp daily to remove nits until they are all gone.