Try changing the tube
If the fluorescent tube is defective, it will cause flickering. These tubes are not expensive and can be easily replaced. If you are careless, allowing the defective tube to flicker, it may overheat the ballast and fail completely.
However, you need to be very careful while changing the tubes because they are sensitive and can be easily broken, resulting in injury.
Check the lamps
In order to open the lens to gain access to the lamps, you need a small ladder and a screwdriver. If you witness a blackened area close to the each end of the tube, it usually means that the lamp is failing and needs to be replaced.
If there is no blackening around the two ends of the tube, try putting the lamps in another fixture. If they continue to flicker there as well, it means they are defective.
A starter is a small silver cylinder that is plugged into the frame of the fixture and is visible on the outer side. The starter can be manually replaced without any kind of a difficulty and is not expensive. Take out the old starter and plug in a new one to check if the tube still flickers. If it does not, you may trash the old starter but if it continues to flicker, it means that there is something wrong with the tube or the lamps.
Change the fixture or the ballast
If the lamp, tube and the starter are fine, you would need to change the ballast or the fixture. If you do not have a small ladder, you may use a stool to reach the desired height.