Why won't my iPod charge?

When I plug it in, it shows that it is connected and that it's charging, but it's not. Instead of the lightning bolt, there's a plug symbol over the battery icon in the top corner when I plug it in. I left it to charge over night and it didn't charge at all. It does this no matter what cord or adapter I use or where I plug it into. Sometimes it'll charge until it gets to like 45% but then it stops. Now it won't charge at all and I'm not sure what to do.. HELP