Sounds familiar and I'm hoping its something simple but I've tried most of the obvious tricks.
After a few hours of Wii Sports the remote stopped recognising most movement, I can still direct the cursor but it doesn't recognise any swings or shaking. I tried slapping it against my palm a few times but to no avail, Later while playing Zelda the +, home and - buttons stopped working. It seems like an actual mechanical fault.
All the other buttons work and the batteries are fine