I assume that the battery is not fully charged?
If it IS fully charged and plugged in it obviously will not charge. As some one said, get the power source checked.
Could be that the battery is kaput. They don't last forever and if constantly installed with the machine running on mains power, they don't last as long as if they are running on mains with the battery uninstalled.
I am currently running mine on mains power with the battery uninstalled for that very reason.