
There are several reasons but the main one being that the chips do move. I had a micro chip reader at a session I was running with John. Of the seven dogs brought which were chipped, on the first dog we located the signal from the chip, but this was not at the shoulder blades where the chips are implanted, it was down by the thigh muscle on the right hind leg. In another the signal was found on the right fore quarter near the elbow, on another we could only get a reading half way down the back, on three the chip could not be found, (this included an imported dog), and on one only we found a signal at the shoulder blades.
I have seen a report from a famous dogs home which regularly micro chips all it's dogs for rehoming, and it said that there were problems with the microchips moving or becoming lost altogether.
Although the microchip companies say the old chips moved and the new ones don't, I recall them saying that the old ones would not move either. Personaly I'll believe they don't move the day a micro chip operator has one implanted in their own necks for a year or two! I really don't see why I should have a foreign body put into my dogs. I tattoo my retrievers.
Anyway this is straying from the point a bit, which was that there are to be changes to the eye testing certificates, and it might become compulsary to register and identify our dogs permanently, which I have no problem with so long as we can chose which method we want to use.