A lot of gaming laptops use all the power coming from the AC adapter when gaming. Since you don't give your laptop model number, I can't break it down. But my laptop has a i7-4720HQ (47W TDP) and Nvidia 970m (estimated 75W TDP). Add in about 15 Watts for the screen and other laptop components, and it tops out at around 150 Watts. Its AC adapter is rated at only 150 Watts. So it can take all day for the battery to charge if I'm playing games (gets a little charge during loading screens and when the CPU isn't running at full)..
Some laptops use even more power than the AC adapter can provide. They get the extra power from the battery. So these laptops will actually slowly drain the battery while you're playing on AC.