Clean both scanner bars. Dampen the cloth with soapy water (it must not drip) and then rub it back and forth frequently over each scanner bar with the cloth stretched over your finger. Finally, moisten the cloth with glass cleaner and go over the scanner bars again.
Now make a scan on both sides. If there are still streaks, go over the scanner bar at the appropriate place with your fingertip or fingernail (or with a magnifying glass and a lamp) and feel whether there is a scratch (e.g. from a paper clip that has been pulled through). If there is a scratch, then the scanner bar must be replaced. Prices and estimates can be obtained from Fujitsu support (0800 673 8187, free of charge).
In very rare cases, an LED in the scanner bar may also be defective. In this case, however, there is a deep black, razor-sharp line about 1 mm wide. In this case, the scanner bar must also be replaced.
In most cases, however, cleaning is all that is needed. Often there are residues of ink on the scanner bar that have not dried and cannot be removed quickly with a duster.
Good luck1