You have to reset your Pokewalker,to do this you have to.
first select "CONNECT TO POKEWALKER" from the menu, and you'll see "save,then go for a stroll". hold down the down button on the D-pad and the button X,then press the button L. once you've read "Caution!" and selected "yes". you'll see the screen on the right, and the pokewalker will connect to the DS system