Best way to find out is to swap one part at a time between printers.
If you switch the drum and the toner and the problem doesn't move away from the device you have what seems to be a bad (probably expensive) issue. But if the problem moves machines you will know exactly which part has the issue.
From that point you may be able to have it replaced by Xerox as defective (Assuming it isn't 3/4 empty and a year old kind of thing)
When I got a messge to replace the toner on my M20i and opened the door there was a ton of toner on the lower right as you face the door. There was so much that it fell onto the floor and was also in the door to the right where the drum is. I cleaned it all up and closed the door. But when I opened the door again, there was still more toner in the mechanism to clean up. I did that twice and then opened the door to take these pictures and there was still some toner -- although a lot less than when I first opened the door and took out the cartridge.
My question is: Does this appear to be a defective cartridge, a defective drum or something wrong with the machine? Except for this, the machine operates normally. I have another m20I and it doesn't do this.