I have this problem from time to time. Especially when other wiimotes that don't normally get synced do, after having synced the wiimotes to another wii or with the computer (GlovePIE) or I have several bluetooth devices running.
What works for me is unplugging the Wii for a minute then press the red button on the wii and then on the wiimote or viceversa. Also try disabling any bluetooth devices that might be on.
Hope it helps.