Check for low voltage at the light wiring with a multimeter or DVOM....could be a bad neutral circuit...white wires are not connected well or connections broken which makes the power ground through ground wire of a circuit which are not as efficient and results in reduced voltages, flickering lights, fans or motors change speeds, etc... Should be 110-120 volts + or - a few volts between the black and white wires ...hot and neutral... depending on how far away you are from a power substation. Check voltage at the breaker panel too if possible ...could be issues with voltage coming into the house or loose /bad neutral wiring connections.

On mine, you clean the tips where they roll in to the connectors as you install the light tube.

They can have some shmutz on them perhaps, or I tap on the light housing. be sure to tell us what fixed yours. they let you enter an update above

Bazza said it. You can get a 4' fixture for $9.

I would look for a new light fitting. You could have bought a new one with the cash you have spent.