Ah, lithium batteries. We love you, we hate you.
Sometimes lithium-based batteries just give out. In some cases, you can rejuvenate them by going through a couple full charge cycles in a row (charge fully, drain fully, rinse & repeat). Lithium batteries die faster if you don't let them drain a good bit before charging them (i.e. don't leave it constantly plugged in); similarly, they tend to die if you leave them without charge for a while. It's almost as if they get "comfortable" with whatever state they're in the most, and die if removed from that state.