It is not necessary to retreat lice unless 1.) you find live lice, or 2.) you are following up with a second treatment as recommended 7-10 days after the first treatment. Some head lice treatments may dry the scalp and cause some itching and irritation.  Further, irritation may also be the result of using many different treatments. A deep conditioner may help restore moisture to your scalp post head lice treatment(s) to alleviate itching and dryness.