Fluorescent lights work by running an electric current through mercury vapor in a vacuum tube which generates ultraviolet light. To make that light usable for illumination it is made to fluores by phosphors that coat the inside of the tube. You may have observed that long fluorescent tube develop a black band as they get old where the electrodes have vaporized and deposited. With compact fluorescent bulbs that are twisted into a spiral I have discovered that the black deposit settles all along the downward surface. My wife complained that she could not read under the lamp over the desk. When I checked It was a 100 wat equivalent CFL but the bulb pointing up was completely dark so little light was coming directly down the light was defused reflecting off the shade. So I traded with the light in my reading lamp which points down or sideways which solved the problem for now.
I have found a manufacturing procedure flaw in bot CFL & LED lights. There is wire for incoming electricity soldered to the center tip of ht screw in base then the outgoing wire is attached to the sidewall of the screw in base.
I bought a box of ten LED corn cob lights and eventually they all failed but with some of the individual strips glowing slightly. Some of my CFL bulbs stop working but when I push on or twist them slightly they will come on and then shortly go off again. Taking a large pair of pliers I worked the base off of the lights that were failing. What I discovered was that the outgoing wire was not soldered to the side of the base. they had simply crimped the base against the end of the wire against the porcelain fixture. With repeated heating and cooling the wire loses contact partially or totally with the side of the base.
Looks like I have a soldering project to get the LEDs working again.