We've had the same problem with our Vaio FZ21E. Basically, you need to unplug the laptop and let it run on battery power pretty much all the time to keep the battery in healthy condition. If you leave the laptop on mains power, the battery will eventually lose charge and will not regain full capacity. So.... you need a new battery. Make sure you run the laptop off the battery unless you are charging it and you should be fine.
It would be nice if Sony TOLD you this in NICE BIG LETTERS in the manual. Or when you log in. But they don't.