Mist likely the connections lost their "springyness" as I like to call it. Try bending the socket out a little bit with a jewelers screwdriver if you can get to it. Another possibility is that the termination has corroded. This can be fixed by brushing the socket with a wire brush dipped in coke-a-cola. If neither of those methods work than one of your internal components in the flourescent ballast has failed, came loose, or corroded. Open up the casing and tighten any loose screws, clean all connections and look components that show melting, popped capacitors, smoke damage, etc. And replace any damaged components like - for - like. If none of the above solves your issue a thourough component analysis and troubleshooting is required and needs meters you dont normally see in the home. I would offer to help with the last step, but I dont own an oscilloscope.
Optimists say the glass is half full. Pessimists say the glass is half empty. Engineers say the glass is twice the size that it needs to be.
