Are you syncing with the red buttons, or using the menu that appears when you hit home using the first remote which prompts you to press 1+2 on the additional remotes? If the later, use the first method, with the red buttons.
(The method with 1+2 buttons is
meant to be one time only, so that if, for example, you go to a friends house and take a remote, you can connect it to their wii without permanently syncing it. This way when you go back home, your remote is still synced to your own wii, so you won't need to reconnect.)