I found this on a related post, but it should help:

Hi,
After posting my message here I started to fiddle around with the wii fit again and went slowly step by step through the synchronisation steps. Voila it works! what I had done wrong was that I did not press the red synchronisation button for the wii console when the blue lights (after pressing the sync button) on the wii fir board started to flash. you must do this when they are flashing. Hopefully you are as lucky as me and it will work for you too.

I have same problem