It appears that some songs in the iTunes store that were added to the store since the end of November will not play on 1st generation shuffles. They transfer to the shuffle fine but will not play. These same songs transfer to other iPods and play just fine. Many people are having these problems (see the Apple discussion boards). The only known solution is to burn the tracks to a cd and re-import them into your library (a royal pain in the butt). And, yes, I have tried all of Apple's recommendations and even went through troubleshooting with tech support over the phone. I was told they'd call me in a few days after speaking with their engineers but never heard back from them.
PLEASE contact apple either by the following link:
OR by calling the support number:
Hopefully, with enough complaints, Apple will be forced to correct the problem. It seems to me (and quite a few other people) that something changed on the encryption/the way they are encoding new music in the store that has made the music incompatible with our shuffles.