I've had that happen, having a prob with several out of one box when the others have been perfectly OK whereas NP whatever with cannulas in other boxes with the same serial number and date. I just reported them to Roche on the phone and they noted it. I think unless they get a whole spate of such reports with the same date or S/N, they don't do anything. I mean there is some wastage anyway - I hadn't pulled the inserter back properly the other day so the whole new cannula shot off across the room as soon as I turned it ready to apply. The sticky folded back on itself and stuck very firmly indeed - so I had to scrap that one, and I'm not too worried about Oh dear they're X quid each and I've just wasted that - cos unless you're getting through a LOT more than you ought to I don't think anyone notices.
You had better tell Roche though IMHO.