Printers check the job description against what is loaded. If the job asks for A4 in tray two and tray two holds photo paper, the printer waits — sometimes silently, sometimes behind a message only shown on the device screen.
Duplex printing adds another dependency. Not every model can print both sides automatically, and manual duplex jobs stop halfway waiting for you to reload the stack.
People often resend the job several times, which fills the queue. When the mismatch is finally resolved, every copy prints at once.
Checking the printer's own display before the computer's queue saves most of this confusion.