Is the charging problem just on the computer or with the AC charger also? If both, you may need to have the techs at the Apple store genius bar check it out.
If it works on the AC charger, then there is a misccommunication problem with your comptuer.
Have you done a reboot of the iPod recently, hold both the power and home buttons until the apple logo appears and it restarts, ignoring the red slider if it appears.
- Try another cable
- Try another charging source
- Inspect the dock connector on the iPod for bent or missing contacts, foreign material, corroded contacts, broken, missing or cracked plastic.
- Make an appointment at the Genus Bar of an Apple store.