Are you using the AC adapter and cables that are bundled with the ScanSnap?
Use only the AC adapter and cables that are bundled with the ScanSnap.
Is the glass of the ScanSnap dirty?
Clean the glass of the ScanSnap.
For details about how to perform cleaning, refer to ScanSnap Help.
An error may have occurred in the ScanSnap
Disconnect all the cables from the ScanSnap, reconnect them after 10 minutes, and then check if the problem is solved.