Definitely the worst part of electrical work. It seems simple enough but then you have a box that lends itself to suspicion because you typically cannot determine if it is good or not. In this case, you are lucky that you have a means to check it. I don't mind electrical work, in fact I like it usually, however when you run into one of those issues where it "should" work, but it doesn't it defies logic and lends to extreme frustration.