Some of the items in the iTunes library were not copied to the iPod because they cannot be played on the iPod

I was syncing my iPod touch to my mac mini, it was previously synced with my windows desktop. I had made some updates to the playlists and wanted to get the updates onto the iPod. Because it was a new sync to the mac mini, it whipped the iPod and then started to re-sync everything, I get an error message saying that 100 of the songs on the list couldn't be synced because they couldn't be played on the device. Most of my music library is ripped from the music CD's I have, mp3 format, and where previously on the iPod and played just fine. I went through all 100 of the songs to check to see if they'd played in iTunes and they all played just fine. I thought maybe it was the USB cord, so I tried a different one, same results, WiFi sync didn't work either. The iPod is a second gen iPod touch running iOS 5.1.1 and I use it for swimming laps. I have a special arm band and headset that are designed for swimming laps, and use my older iPod touch just in case something happens and water gets into the water tight armband.