Check the following points to resolve the issue:

• Make sure the two green levers inside the back cover are pulled down to the envelope position when you print envelopes.
  

  After you have finished printing, reset the levers back to their original positions. Pull up the levers until they stop.

  

• Make sure envelopes are printed with the back cover (face-up output tray) opened.
• Make sure envelopes are fed only from the manual feed slot.