I had this same problem the other day and could not figure out what was wrong. I finally rubbed my index finger over the metal inside of the tail light where it connects and put the bulb back in a it worked! I was just about to give up. I think there must have been some corrosion or dirt on the connection and by wiping it off, I allowed for a better connection. This may not work for everyone, but it is the quickest and cheapest thing to try first. Good Luck.