Generally, if the laptop is plugged in but not charging the battery properly, it is a sign of a problem with the DC power jack. Other common signs include a flickering screen that can occur when the power switches arbitrarily between the battery and DC.
The DC power jack is a weak spot on many PCs and is held in place on the motherboard by only three or four pins. If one of the pins is out of place it can cause overheating and even smoke to come out of the jack. Having to jiggle the plug in the jack to make it work is another sign that the connection is faulty and needs servicing.