Have you checked to make sure the glass plate is clean? also the underneath side of the glass, make sure there is not a bloom on it.
Check manual for calibration of scanner, and also try adjusting brightness/contrast from scanner software.
If non of this helps, how old is the scanner? maybe the lamp is loosing it's brightness.
Try this excellent site for cleaning/maintaining, and repair of scanners. The link was provided last week by one of the forum users, unfortunately, I can't remember who. click here