The machine does not use the toner directly from the bottle, it goes thorught a developer chamber filled with developer powder. Then the toner is charged and can "jump" over to the latent image of the drum.
In this developer chamber, there are some small gears (2-3 tiny gears) that may be worn out. Some developer may have entered the "gearbox", and the deveoper powder is very abrasive.
The teeth of the gears will eventually stop to contact and mesh up. The internal parts of the developer changer stops to revolve, and there is no toner mixed with the developer.
You can see that there is a build up of toner, but it does not enter the developer unit. These gears have to be replaced, and I allways change the developer powder at the same time.
These gears must be replaced it they are worn. The image shows a brand new part that contains the gears.
This is the side of the developer unit where toner enters the developer unit. With defective gears, the developer unit can not pull toner inside of the unit, and the toner builds up in front of the unit.
The gears are cheap, but will often result in the need of new developer powder - and that is expensive.
If print quality does not have to be too good, one can use the old powder and save money. Some of the powder is lost during use, and leaks. There may be some dust in the developer too.
In order to get high quality several parts may need to be replaced.
It is quick and easy to change these gears, just a bit dirty job.