Docu Print CM305 df Faint Ink

Hi There, 

We have a Docu Print CM305 df and recently it has starting printing anything black very faintly (except pdf documents) first we changed the ink cartridge, then we followed the cleaning procedure within our manual and we have still had no luck can anyone suggest something we can do to stop this as we need to be able to print things in black promenently. 

Thanks in advance.