Why cant i get rid of lice??

Alright, so ive had lice for around a month or two, and ive used the nix “shampoo” cream (and a lot of it too) atleast 3 times all one week apart and my head will feel great for 2-3 days after treatment and then get all itchy again after that point. I cannot find nits in my hair no matter how hard i look, and the only time ive ever combed out an actual lice was before the first treatment. I dont have a lice clinic near me, and i dont have anyone to comb out nits. Any advice?