Try these... maybe they'll work
update ipod and itunes for latest features
Have you actually played the songs on your itunes
Do any of the songs have anything in common, purchased after a certain time, other computers, account info change.
Double click/right click the song, and see if you can enter you name/ID and password on the individual song.
Will they burn to a Disc, if so burn them, then download them again.
Try making a playlist on iTunes, then specifically make sure that playing is checked off to download onto to your ipod.
as a last resort, de-authorize all computers, and only authorize the one your syncing and/or loading songs with.
if all else fails... try repair disk in Utilities Application
Open Disk Utility (in /Applications/Utilities).
Select your Mac OS X volume in the left column, then click the First Aid tab.
Click Repair Disk Permissions
See what happen, maybe something will work...
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