Why does my MacBook Pro say "Not charging" when plugged in?
Some obvious causes:


Need to wait 30 to 60 seconds for the SMC to read the battery and decide what to do
check for pencil lead/debris on the mac end of the magsafe
check for pins stuck down on the magsafe cord end of things
The SMC needs to be reset on the Mac
The battery, all adapter or charging circuitry itself is faulty or detecting something that causes a pause and the mac might need service. (This is exactly what your mac will do if it needs an 85 watt adapter but you've somehow gotten your "friend's" 65 watt adapter - if your adapter isn't putting out enough power, the mac can only run and not support the CPU/GPU load as well as the charging circuitry. Similarly, if the battery is drawing more than designed, the mac will prefer to run than charge and tells you this in the not charging message.)


Due to the way batteries work, they can be out of tolerance / spec at only a specific portion of the entire voltage range. The sensors will correctly detect “good battery” until you reach that one point where the voltage drops too fast and shows aging or a problem with the cells.

Do a calibration run at your earliest convenience if your battery doesn’t continuously self regulate and calibrate. Next, think about a visit to the genius bar or other Apple Support / service desk if the problem continues. Take your wall adapter with you when you go as it can be checked with the mac together.