Two-sided printing is not working
Start by checking whether this printer can print on both sides automatically. Then check the print setting before changing printer software.
1. Has it printed both sides automatically before?
If yes, the setting may have changed. If no, check the exact printer model—some printers only support manual two-sided printing.
2. Windows
Open the Print window, choose the correct printer, then open Printer properties/preferences. Look for Print on both sides, Two-sided, or Duplex.
If that option used to be there and disappeared, the printer may have been added with the wrong setup or driver.
3. Mac
Choose File → Print and look for Double-sided. If the printer supports it, turn the option on and try a short test.
4. Try plain Letter or A4 paper
Some printers will not print both sides on envelopes, labels, heavy paper, or certain paper sizes. Plain paper is the easiest test.
5. If the printer does not support automatic two-sided printing
You may still be able to print one side, turn the pages over, and print the other side. Follow the program or printer instructions so the second side is not upside down.