Like many others I received the letter from Virbac last summer warning of potential problems with the chips implanted in my 2 cats. I had them them checked immediately and both were fine.
Recently I installed a microchip cat flap but try as I might I could not get it to recognise either chip. Since Biggles had a vet appointment anyway I got them to scan him and they couldn't find any chip. They even put new batteries in their scanner and tried again but no joy.
They asked had he been near a strong magnet but unless extra-terrestrials have put a force field around our home then the answer to that is no.
I think that at the time of the letter Virbac stated that the chips failed soon after implantation if they were going to fail at all, but in light of my experience I believe this to be incorrect (mine were implanted in 2011).
The vet has re-chipped both cats (they couldn't find Bunty's either) but I'm still waiting to hear whether Virbac accept liability and foot the bill.
I would advise anybody who received one of these letters to have their cats checked again next time they're at the vets. In fact its probably good practice to have all chips checked annually at booster time. This is especially important if you're taking your pet abroad as we used to do with our previous cat Korky.