Have you tried blue Ivermectin Pour on to treat the lice? It is a topical treatment that is used by applying it down the back along the spine once every 10 (ten) days. Also important would be to remove any and all bedding material each time on the day they were being treated.
What type of lice powder did you use and how often were you applying the lice powder?
What type of lice powder did you use and how often were you applying the lice powder?