iPod 4g charging slowly

My iPod 4th generation has been charging really slowly for about 2 weeks. I'm using the adapter the plugs into the wall and that's not the problem I think. It still charges but it doesn't charge fast. I might plug it in at 11:30 pm and at 7:00 am it's not even halfway charged. The battery gets used really fast and I don't jnow what's wrong with it. I've had it for a little over a year. Please help!