Depending on the application you are running, there are two reasons why this occurs:
If you are running scanadf, the source is set to Single Page.
If you are running XSane, the ADF Modus (located in the XSane setup window) is set to Multiple Pages, but the source setting (located in the XSane main window) is set to Single Page.
If you are running scanadf, type:
; or
If you are running XSane:
In the XSane main window, choose Multiple Pages from the drop-down menu next to the scanner icon.