While scanning some dust can stay on the glass lenses., it has no effect but with time it can produce images with bright vertical lines (usually pink or green). A little cleanup of the lenses and you can continue scanning. Also, if the cards are not scanned as easily (speed or cards tend to get stuck) its time to clean the rubber wheels.
To clean your scanner you need to use a clean cloth with rubbing alcohol and clean the wheels, skate and lenses carefully. Wait 30 seconds and close scanner.