I've had this problem occasionally, too, and it has been caused by either fingerprints or scratches. I think dust inside the wii can also cause it.
For fingerprints, I just clean the disk with warm water and a soft cloth, not paper towel. I use vinegar or a little dish soap in the water when the prints are stubborn, but never anything harsher. IMPORTANT: Do NOT rub in a circular motion around the disk, but instead rub in straight lines from the center to the edge. The reflective side is the one the player reads.
For very fine scratches I have used toothpaste which I found recommended on the web, and it worked for a while, but you do have to be extremely careful to not scratch the disk yourself. Ultimately, I found the best way to deal with small scratches was to take it to take the disk to a place where they could polish the scratches out on a special machine. GameStop used to do that, but around here (Philadelphia, PA) they don't anymore. I found a place that specializes in transferring video to DVD which also had the polishing machine. It only cost me $2 per disk which is lots better than replacing the games.
I also invested in a Nintendo head cleaner for the wii which I use once in a long while. Not sure if it helps or not, but it makes me feel like I'm taking care of the wii :)
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