Tester's answer is the more likely, but here's something else to keep in mind.
Newer flappers have an adjustment where you can turn them to flush faster or slower. This aligns an air hole between straight up (faster flush) to the side (slower flush). Air bubbles out of the hole and water gets sucked in the bottom until it's heavy enough to fall closed.
So now, if you have one of these designs, you may just need to twist it to the appropriate setting (there's usually some numbers written on it and you can feel a distinct clicking as you twist it). But I can also see some debris getting stuck in the drain hole causing water to stay in the flapper, so that it never resets to be full of air. If that's the case, clean the bottom off, or as Tester says, replace it.