This has come up before and I was about ready to replace the ballast. But others have posted that sometimes corrosion could be preventing good contact between the contacts on the long bulb and the fixture so I worked the bulb in and out of the fixture several times and the fixture is now fixed. No new ballast was needed.
Many 1.5 inch diameter 4-foot "F40" "Cool White" fluorescent bulbs are not "true 40 watt" but "energy saver F40" which is actually 34 (sometimes
35) watts. These are generally crankier. True 40-watters are usually "deluxe cool white" (which have less light output) or triphosphor (color-brightening - usually more expensive). I consider it worthwhile to get true 40-watters in a "triphosphor" version such as Philips Ultralume or Sylvania "Designer" or "Interior Design" in order to get good performance, full light output, and color distortions mostly in the direction of making colors more vivid when using 4-foot 1.5 inch diameter bulbs.
If the bulb size is not 4 feet by 1.5 inch diameter, the above has much less relevance.
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.