First make sure that your bulb is turned solidly into the socket. On a flourecent bulb, it will slide into the slots and turn 90 degrees to engage completely.
Also, on a flourecent fixture, you will find a little white round cylindar device - that is the starter. Its failure is one of the most common problems in a light. Starters are very cheap to replace - they just plug in and out. An easy place to find one is in the pet store fish section - next to the new bulbs.
Finally, you may need a new bulb. I replace all my bulbs once a year, at least, for they dim after an amount of time and no longer give the proper amount of light. I cannot visually determine the output, but my light meter does. A flickering bulb may be reaching the end of its life.
See if any of these suggestions helps.....