I also have this problem sometimes, and also get the plugged in not charging message in the taskbar/when hovering over the battery Icon. Mostly I detach the charger and wait a little, than attach it again, If this doesn't work I reboot it. Funny thig is, that this is happening while it is charging. Like I plug it in and it charges, I go away and come an hour later back and suddenly it's not charging anymore.
My original power supply would indicate that it was plugged in, but say not charging.
I would unplug and plug it back in and all was fine.
However, eventually it gave up. So I took the supply back to the shop - John Lewis UK - they confirmed and gave me another power supply, which has been working ever since. Occasionally giving the same 'not charging' message. Unplug and plug back in and all is fine.