Make sure you are at the latest firmware, 2.2, you can check in iTunes.
Next, hard reset the iPod.
Next, you can try downloading a new app, this sometimes corrects the memory corruption that a reset won't.
If all else fails, delete the apps and redownload them from the AppStore. If they are free then you can download them as many times as you want to. This will fix the issue. Sometimes the apps get corrupted and need to be reset.
I have only had this issue twice and one time downloading a new app fixed it and the second time I had to redownload the apps that did not work.
