Replace your lights!
It doesn't matter what type you have, screw in, tubes, horseshoes or whatever other fluorescent you might have make sure to replace them at least once a year. You won't notice it but the light gets dimmer and dimmer over time and it will effect your plants. They also lose their effectivenss as far as spectrum, lumens and their CRI. Remember that if you are overdiving your normal output fluorescents that it cuts their effectiveness times in half. E.I. If you 2x overdrive then you need to replace your bulbs every 6 month, 4x overdriven, replace every 3 months. etc. Overdriving them burns them out much faster and the extra heat can effect the specturm and CRI a lot faster.
The only bulbs you really don't need to replace are power compact bulbs until they actually burn out. If you do have one burn out and the others don't, I would still either order them to have them on hand or simply replace them all at the same time.