Please check the following points:

• Check that the recommended paper is used.
  To see what is the recommended paper, click here.
  To see what kind of paper you can use, click here.
• Check that the media type mode is selected properly.
• Check that the release lever of fusing unit is selected properly.
• Replace the fusing unit.
  To see how to replace the fusing unit, click here.

If this problem happens when you print on thick paper, set the printer to thick paper mode through the control panel or the printer driver.