It will be better if you upload some images with the issue.
Have you replace the drum cartridges with new ones or you just swap between them?
This machine does not have a Laser Unit, it have Led Print Heads (one for each color) and you can clean them with the orange Rods. If the lines are discolored it means it could be from there, but if they are colored it is from drums or from the IBT cleaner.
If you cannot see issues on the drums this does not means they are in good conditions. You can diagnose them if you switch between them and if the problem move to another color it means that drum is defective, or to replace them with new ones.