Much of this depends on how much you move the laptop. Pluging in and unpluging expands the contacts on AC adpt. at batt. over a long period of time. If not that the batt. contacts INSIDE unit are dirty/burned/corroded, etc. If that be case it's time to exercise your warranty if it still applies.

Do you use this a lot to travel? does it move around a lot?

one of the battery connections inside the case could have come loose? If you are skilled enough you could take off the case and make sure all of the wires are making proper connection.