Android :: Update Item Position On Map Doesn't Work?

i have an android app, with a actitity, that shows a map, with a position. This possition is updated on sharedPreferences with a service... ok, that works fine. I need that my activity implements runnable to be a thread that read's the position of sharedPreferences each 1000ms and then update the item position on the map... but it doesn't works. As i saw in debug mode, my thread doesn't work fine, ONLY ENTER ONE TIME on RUN() method... i mean that never enters into RUN() method again, only one time, cause this, the positions doesn't be updated on the map. What is wrong? why this thread doesn't work fine?

public class Locate extends MapActivity{

private TextView userText = null;
private TextView permissionText = null;
private TextView lastUpdateText = null;
private Button locateButton = null;.............