A possible cause may be that the amount remaining in toner cartridges is running low. Check the following.
Are white areas lightly colored?
NOTE
If you set to <On>, print density may be lighter.
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