I had a call from a customer about some of there lights not working. When i went to check them out i had voltage at the fixture so i replaced the bulb but nothing. So I went to another fixture on that run that was out and when i went to take the cover off the light came on. I went to another and tapped the fixture and it came on. This problem came up on several fixtures. Has anyone came across this and what is a possible solution? I figure there is something loose but I don't see how this can happen on so many.