Exact same problem!
I just found this thread, cause I have been experiencing the exact same problem...
I tried everything, point the nunchuk down, shake it, swing it you name it... I guess mine is busted as well.
All the functions work properly on all other games, but I did notice that when playing Wi Sports' Boxing my left hand is almost always pointing down, meaning I can only do body shot with it. Check if yours is like that as well.
Anyway I might have to buy another nunchuk, good thing it sells separetely.
Let me know if you find something, I tried to find a workaround in the game, but no luck.