Why won't my battery charge?

I got a Nokia Lumia 635 about a month ago, and all of a sudden when I woke up this morning my phone had less battery then it did when I went to bed last night and the battery level just keeps decreasing and I don't understand how when its a brand new phone.

You might have an app that's not going to sleep. Go to the battery saver app and see what app(s) are draining your battery the most.