As with many devices that use a rechargeable battery, when a charger is plugged in, the device (In your case, your computer) will take the power from the charger instead of charging the battery, which is common among many devices, including mine.
If you want to charge the battery, you will have to power off your device (computer), place it on the charger, and let it charge until full
''Note: Your computer must be fully shut down or in hibernation (the process of the computer saving everything that you are doing and shutting down so that it returns when powered on, like sleep mode but it fully shuts down the computer). Placing the computer in sleep mode may not allow the battery to charge.''