A possible cause may be that appropriate paper is not used or the amount remaining in toner cartridges is running low. Check the following.
Are you using appropriate paper?
Check usable paper, and replace with appropriate paper.
Do ghost images appear in the same page?
NOTE
If you set to <On>, the print density may vary depending on the ambient humidity.
Do afterimages appear when you print on large size paper after printing on small size paper?
NOTE
If you set to <On>, the printing speed may be slower.
Is the amount remaining in toner cartridge running low?
Regardless of the displayed toner level, the toner cartridge may have reached the end of its lifetime due to the machine's environmental conditions and the deteriorated materials in the cartridge. Replace the toner cartridge. Replacing the Toner Cartridge