The way a copier/laser works is easy. You need to see where it is failing.
The way it works is Charge the drum, laser writes a character onto the drum thereby changing the charge, then the toner is set at a different charge to the characters that have been written and the ink is transferred to the drum, the paper is then feed through and itself is charged to attract the toner ink from the drum onto the paper, then as paper goes through the fuser the ink is plastic beads which is melted into the paper.
1. So see what is not working.
2. You do not say if the the bit missing is towards the end of page or left/right hand side of page.
3. Start copy and as the paper is part way through open the copier which ofcourse will stop the copier and see what the image is shown onto the drum (you will see the characters like they are on the paper ), if the characters are missing it could be a dirty window onto drum, remove drum and clean window, if window is clean swap the toner unit.