here is some info and link - do you remember if the vet did the implant in the neck area

as this states here as the neck area can be key in what is going on

is the chip still there can you feel it?
http://en.wikipedia.org/wiki/Microchip_implant_(animal)
Implant location
In dogs and cats, chips are usually inserted below the skin at the back of the neck, between the shoulder blades on the dorsal midline. Continental European pets may be an exception; they get the implant in the left side of the neck, according to one reference.[1] The chip can often be manually detected by the owner by gently feeling the skin in that area. It stays in place as thin layers of connective tissue form around the biocompatible glass which encases it.