Shania is right.. it seems that they don't knock on doors as often as they used to. Blondie gave a real good suggestion.. check with your neighbors. It might be that they just really haven't been in that neighborhood in a very long time. It is also possible that they had really bad experiences in that nieghborhood. See, there's a particular street not far from where I live where the JWs NEVER go door to door. I don't know the details, but I know that they were threatened there by several people a few years ago. Instead of a do-not-call-on door, they have a whole do-not-call-on street. True story.
Oh, and check your doorbell, the mice might have gnawed on the wires, perhaps it doesn't work.
If your doorbell works and the neighbors get the JWs to visit them often, then you'll know where you stand. Poooooor thing. I'll come and visit you, how's that?