Check the following points to resolve the issue:
NOTE: Illustrations shown below are from a representative product and may differ from your Brother machine.
Make sure the two green levers inside the back cover are in the up position. Pull up the two green levers until they stop.
Make sure the Media Type setting in the driver matches the type of paper you are using.
Select 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, select Env. Thick in the media type setting.