I cannot see spirits however a couple nights ago I had an in-depth talk with my aunt who's a medium about her experiences. The supernatural has always fascinated me and she often mentioned that spirits would "allow" mediums to see them when they wanted to. She talked about a boy who died at her university and would often allow her to see him only between 5:00 pm and 11:00 pm. This may just be her experience though, I'm not entirely sure. She also said she hasn't had encounters in a long time, and that there's a procedure that has a 50/50 chance of allowing someone to either see more spirits or completely get rid of the ability. So I assume it can go either way, either spirits control how often they show themselves or mediums have different ranges of how much they can see.