Print jobs are always sent to manual feed tray

My housemate bought a mono laser printer a few days ago for their office. An issue arose was that some documents were always sent to the manual feed tray of the printer for printing. It would be a nightmare when the document is long, since the printer requires one paper at a time. He claimed they never had such problem before, and drew a conclusion the new printer was a crap.

Ok, I admitted that their new printer was cheap, may not be capable of their need. Firstly, I asked him to check printer configuration. It seemed all settings were correct and the document was sent to another tray other than the manual tray. Then I recommended them for a refund. Unfortunately, the receipt was damaged in one day after installation.

I had no idea what happened but searched the Internet and no similar problems were found relating to this printer. Did it imply there was a bug in the printer driver? I went to his office and checked, it was the newest driver which should be fine.

Suddenly I realised that one could set the paper source in the word document too. I opened the window for Page Setup in Word, and switched to Paper tab. Problem found! The paper source of both first page and other pages were set to manual feed. I changed it to automatically select and it worked.