Your printer only prints on one side even though you selected two-sided. Usually it's a driver setting, or the model only supports manual duplex.
Work through these in order; most people are fixed by the first two or three.
In the app's print settings, choose Two-Sided / Print on Both Sides (and pick the correct binding edge). Set it as a default in the driver to make it stick.
Some printers are manual-duplex only — they prompt you to flip the stack halfway. Check the spec sheet for "Automatic" duplex.
Auto duplex needs standard sizes/weights; very heavy or small paper disables it.
Reinstall the full driver if the duplex option is missing or greyed out.
Browse step-by-step setup guides, error-code fixes and driver help for every major brand sold across the USA & Canada.