Worn Developer does not typically light images on partial copies it causes lite washed out copies across the entire page. Most likely you have dusty, dirty optics, dirty or worn charge corona/grid or as solution #1 states, a dirty laser **** glass.

Remove the copiers main plate glass by removing the five silver screws, remove the covers where the screws are located then remove the glass by lifting upward. clean all mirrors and the exposure bulb and all reflectors. Make sure the main power switch is off before cleaning with lint free soft cloth.

Clean you **** glass. it is a thin glass just underneath the toner and to the left of the drum. if toner is on this glass it filters out the laser and copies will be lighter.

The developer needs to be changed in your copier. It should be done by a service technician since the the toner developer sensor has to be recalibrated and all developer has to be removed out of the machine including the mag roller. If it isn't done properly the problem will continue.