Make sure that it is inserted and seated properly. The tray is empty. Load the type of media requested. The wrong size media is loaded. Load the correct size media.
Paper is loaded and transparency was requested or vice versa.
Load the type of media requested. The printer and the application you are using have a different type of media selected. Verify that both the Printer Set-up dialog box and the Application in use have the same type of media selected. Media is not being picked from the tray. Clean the pick rollers as described on page 5-12 of the Operator's Guide.