Check the following points to resolve the issue:
 

1. Make sure the two gray levers inside the back cover are in the up position.
2. Make sure the media type setting in the printer driver matches the type of paper you are using.
3. Choose Improve Toner Fixing mode in the printer driver. If this selection does not provide enough improvement, change the printer driver setting in Media Type to a thick setting. If you are printing an envelope, choose Env. Thick in the media type setting.