Did you scan documents written in lead pencil?
Scanning documents written with a lead pencil may contaminate the rollers.
To prevent the next document that you intend to scan from becoming dirty, clean the rollers after scanning documents written in lead pencil.
For details, refer to here in the ScanSnap Help.
Are the rollers or glass inside the ScanSnap dirty?
Clean the inside of the ScanSnap.
For details, refer to here in the ScanSnap Help.