Itunes/ipod issue sync issue

I need some help. I have 4239 songs in my library on my hard drive (and also backed up on a portable hard drive). When I plug in my ipod to sync, it won’t sync all the songs though each song is checked. I restored my ipod to its orginal settings and started to sync again. it recognized all 4200 songs but when i woke up this morning, there was an error message that said it could not recognize my ipod. I only got 2692 songs synced up. I have plenty of space on the ipod. I also spent 2 hours on the phone with apple support and know that the everything should work.

does anyone know what is going on? is this common? how can I fix? thanks in advance for your help.

iTunes 7 shows the iPod on the left side - I have good luck manually dragging and dropping tracks from the music list on the right over to that icon on the left. I don’t use the Sync command.