Some laptops have a thermal sensor when the system gets too hot, which prevents the battery from charging to prevent damage. My laptop has this, and when playing heavy games, the battery won't charge but the charger will supply most of the power to the system. When I let the system cool down, or play a game that is less demanding, the battery will recharge normally. If your going to play a demanding game on your laptop, its best to stay plugged in, or recharge the battery fully before playing.
Hope this helps.