Seeing things from the manufacturer's side, when you've gone through everything and have not found the source of the product failure you have only two choices: make another lot of product (using what you believe are the best raw materials and best production methods) or stop making your product (deal with unfulfilled orders and lay off employees).
I've been through this process before (product failure but no source of the failure found) and I can tell you it is very unsatisfying for the manufacturer to not find a cause. Without finding a cause you're just waiting for the product to fail in the same way again; sometimes it does and sometimes it doesn't.