HP Deskjet 6800 Printer series - (Macintosh OS X) User's Guide

Note: Do not press the buttons on either side of the duplexer when installing
it in the printer. Use the buttons only when removing the duplexer from
the printer.
12.1.2 Automatic two-sided printing problems
Option for automatic two-sided printing cannot be selected
Verify the following:
The duplexer is attached to the printer.
A supported paper size is selected in the Page Setup dialog box.
A supported paper type is selected in the Paper Type/Quality panel of the
Print dialog box.
For a list of support paper types and sizes, see the duplexer specifications.
Only one side of the page prints when the two-sided printing option is
enabled
Verify the following:
The correct print options are set in both the printer software and the application
you are using.
The photo paper is not curled, if you are printing on two-sided photo paper.
Photo paper should be flat before printing. See storing and handling photo
paper for more information.
The paper size selected in the Page Setup dialog box is supported by the
duplexer.
The paper type selected in the Paper Type/Quality panel is supported by the
duplexer.
See the documentation that came with the duplexer for a list of supported paper
types and sizes.
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