batteries can only be recharged a certain no. of times. If they are the older type you could have the memory effect. That is where if you only give it a little charge eventually that is all it will take. Try leaving the laptop connected to the mains overnight. If that does not work find a way of completely discharging the battery and it may then take a full charge. With the battery removed from the case could you use a voltmeter to determine the voltage it is putting out.