I would trust their assessment. I'd bet the one that works now is in it's way out, seems to be the way they die. It works, but is likely outside their test performance spec, and I don't think there's anything repairable outside of cable replacement. You can certainly get those returned and continue to use the one that works. Yes, it'll be a toss-up matching its sensitivity to another, OTOH no reason you can't compensate.