Depends on what you are doing.
Windows,Mac,Android, iOS, cloudprint, USB all should duplex, but if only one is having an issue we need to know which you are using.
My first guess, assuming PC/Mac, would be that it is application specific (Only happens in MS Office, or Adobe CS/CC for example) in which case it will be an application over-ride we need to change.
If always happening in every application it would be the print driver that needs to be redone(complete remove and install again after a reboot and make certain it is the right one and not OS supplied (Class driver in Windows, Airprint/ Generic PS in OSX)
If it happens on print via USB direct at the printer then tray 5 is broken and no amount of settings changes will fix it.
I was having the same problem. I solved my issue by adjusting the print options through windows for that printer. Once the Printing Preferences window opens up. Click the Down arrow for "PAPER" and scroll to "Select By Tray" and click on "Bypass Tray". Then click on Apply. Load your paper onto the Bypass Tray and press ok on the Printer Keys. Then print your document as usual.
I did notice that I can leave it on the bypass setting in windows and it automaccally pick from Tray 1 if no paper is detected in the Bypass Tray.
Hope this helps!