Contact sensor factory reset not working

My contact sensor (1st gen) has been showing as ‘tampered’ straight out of the box. I follow the factory reset instructions, holding down the tamper switch (for ages) and reinserting the battery. The instructions say to hold down the switch until the blue light stops flashing, but it never flashes in the first place. It has one brief flash when I reinsert the battery and that’s it. I’ve tried every combination and length of time holding down the switch with no joy. Seems faulty given it was already tampered straight out of the box?