Thunderbolt display not charging MBP battery I have a mid-2012 Retina MacBook Pro and two Thunderbolt displays purchased in mid-2014. The MBP sits plugged into one of the Thunderbolt displays almost all the time, unless I'm using it away from my desk. 

Lately, I noticed that the computer's charging behavior was odd:


The menu bar's battery indicator did display the "charging" lightning bolt;
The percent charged number did not change over the course of many plugged in hours;
The indicator light on the MagSafe charger did not turn on.


Some power was clearly coming through, since the battery was not draining; it was just also not charging. Restarting did not solve this, and neither did resetting the SMC (my go-to move for weird power or peripheral glitches). But: plugging the laptop into the other Thunderbolt display did solve the problem. 

Thus it seems there's a problem with that particular Thunderbolt display. How do I troubleshoot it?
I just did what AdamJ said, took my magsafe adapter off and clean it up with a toothbrush to remove dust. It really worked and now my thunderbolt display is charging my macbook pro! :)