Have you tried hard resetting the iPod? hold the sleep and home buttons for approx. 10 seconds until the Apple logo is shown with a black background like it's restarting. Sometimes the software needs a "breather" and doing a hard reset clears memory and restarts everything; that might trigger the charge. (this does not affect any data on the iPod)
Disclaimer: this is a total guess. Sometimes my iPod doesn't charge at 80% and it eventually fixes itself.