Did you scan a document that can be damaged easily such as a photograph?
You can protect documents that can be damaged easily such as photographs by using an optional Carrier Sheet.
Place one sheet at a time in the Carrier Sheet and perform a scan.
After scanning the Carrier Sheet, crop the saved image to a desired size.
For details about how to crop images, 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.