As Dave mentioned, adjusting your media type setting to a heavier option will typically resolve this type of issue. The manual for the Brother MFC-L8900CDW lists a few additional suggestions that may improve your results as well:
- Make sure the two gray levers inside the back cover are in the up position.
- Look for a setting in the print options called "Improve Toner Fixing" and enable it.
- If you're still experiencing poor toner adhesion when using the Labels media type setting, try the other heavy media type settings like Glossy Paper, Thick Paper or Thicker Paper.
- If you're using the draw tray to load the labels, try using the multi-purpose tray (MP Tray) instead. This allows for a simpler feed path through the printer and can have a positive impact on toner fusion.
I've provided a link to the printer manual below in cause you need to reference any of the details mentioned above:
https://download.brother.com/welcome/doc100693/cv_mfcl8900cdw_use_oug_a.pdfI located most of the information related to troubleshooting toner fusion problems on page 576 in the manual.
I hope this helps. If you need any additional information please just let us know.