elisa,
Mushrooms like low to medium lighting and some current. It sounds like you have only one flourescent tube, I'd guess 30 watt, over your tank...right? If that tube is as old as your tank, it's probably not enough lighting even for shrooms (if you have to replace your bulb, you may want to consider, at least, a 2 tube hood so you can supply 60 watts to your tank) . Shrooms also like good water quality. If your trates are high, that may cause them to close, too. Mushrooms also need calcium. Normal water changes can renew calcium in your tank but that's often not enough. You may want to purchase a calcium test kit and if your water doesn't test at 400-450 ppm, you'll need a calcium supplement as well.
I hope that helps.
Jim