I have the ipod classic 80GB and my friend just gave me a top 1000 of all times, I added it to my itunes library and hit synchronize. But nothing is being added to my ipod... It's far from being full I still have about 50GB of free space.
There's a million or so troubleshooting things you could do. One of them being the ipod thread.
Look up the rule of 5 r's