Another way to get an apparent 0 CFU datum--not likely your problem, but worth mentioning so that you can rule it out--is to have a tested material turn out to be non-biocompatible, and in fact sufficiently toxic (or using other language, anti-microbial) that it cannot be evaluated using normal methods.
Materials containing silver or copper at their surface, or sometimes other metals, will do this at times even if not designed to do so. So might a material that contains significant levels of certain other elements, or of course intentionally antimicrobial additives.
So, less favorably, can materials containing certain flame retardants, heat stabilizers, preservatives and flexibilizers.