Easiest choice is to buy a new Nunchuck. They;re only $20 in the US which is not a lot of money to ask for. While you're picking that up, get a Wii Remote too if you have some spare cash. Sets are always better than one more than the other.
2008 Jaguar XKR Portfolio
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you could try taking it apart and cleanning it and see if its just dirty just a thought though