The Nunchuk attachment is not responding.

Make sure the game being played uses the Nunchuk. Look for the icon below on the packing and read the game's instruction manual for more information.
Clear the syncs on the console.
Try a new set of batteries. Sometimes weak batteries can cause these types of problems to occur.
Reset the Wii Remote.
Use a game mode that utilizes the Nunchuk to test whether it is responding. In Wii Sports, the Boxing game can be used.
If possible, try the Nunchuk in another Wii Remote, or try another Nunchuk Controller.