iPod Touch will charge from computer USb, but not from AC outlet

I have an iPod Touch. If I plug it into the AC outlet, with a AC/USB power adapter, the iPod will charge to about 20%, and then stop charging. However, if I plug it into a computer USB outlet (using the SAME USB cable), it will charge fully to 100%. Electricity is electricity, no matter where it comes from, so why wouldn't it charge from an AC outlet?