It's not a secret, but you need a technician, because you need to remove several components of the toner dispersion to see what is damaged or clogged. The idea is that the toner must reach the development tank where the toner concentration sensor is located. If the toner does not reach there, the machine logic states that the toner cartridge is empty. In principle the cartridges that the machine has declared empty, will be able to be used and will show the correct percentage after the problem is solved. I am sorry for your problems, but I have not heard and have not at least such defects so far.