Sometimes the black plastic tip of the power plug snaps off inside the power jack. Sometimes it will get stuck inside the power jack and stops the plug from being inserted fully. Although it is a little bit of a fiddily soldering job as one of the pads is a tight squeeze it isn't that hard.
A suitable replacement part for the power jack is the Digikey (or other suppliers) Part # PJ1-022-SMT
rkdavis from the #zipit channel has written a short blog post on his experience replacing the power jack