PAM does not kill the mites on the snake. PAM kills the mites between feedings when they leave the snake to molt or lay eggs. The population won't decline much until the life cycle of the mite is broken.

"I cleaned the lid of the enclosure this morning. There were several mites, some of which were still alive. " If there were live mites on a surface that you had treated, then the surface quite possibly wasn't treated effectively enough.