Why is my iPod having trouble charging?

Koala, the first thing that I would do is try to charge it on a AC charger and see if that makes a difference. Make sure that your USB cable is good and that your dock connector is in good working order as well. Check for bend,broken pins, and clean to make sure there is no debris (pocket lint) that prevents it from charging. If that does not help it any, I would replace the battery. After that I'd try to see if it can be placed in either disk mode, and check the hard drive to make sure that it is working okay. Also, you want to try to place the iPod into Diagnostics mode to see if it runs all the test. Right now I would concentrate on two things, the battery and /or the logic board. Hope this helps, good luck.