I would recommend calling in a service call on your Xerox. The issue could be related to the contacts inside the machine or possibly even a defective toner. Your best bet would be to contact an Authorized Service Provider or Xerox Support.
I changed the black toner in my printer, but it appears that the black toner is empty... i insert and remove many times to work... sometimes the printer detects my toner (98%) sometimes shows NO BLACK TONNER..