The self clean coating is produced on state of the art coating plants, and rarely is the source of any issue. More likely are external factors, such as contamination on the face of the glass, stopping it working. This can be many causes, from tree sap, to heavy dust or silicone (during the installation process).
Watch a useful video, illustrating the affect on the glass when the external face has been contaminated with silicone. The glass no longer 'sheets' but forms 'beads'. This is one of the most common causes of issues with installations incorporating self cleaning glass.