HP Deskjet 6127 Color Inkjet Printer - Manual (Page 138)

only one side of the page prints when the two-sided printing option is selected
Use the following solutions:
●
Print the document using the manual two-sided printing feature.
●
Use a media type and size supported by the duplexer.
paper jam occurs inside the duplexer
Follow the instructions for clearing a paper jam in the printer is not printing section.
138