Has anyone experinced this? You have SMS opened in the middle of a chat with someone. You press the power button to turn your screen off. Later, you come back to see that the person sent you a message, but you didn't hear the notification sound. That's what keeps happening to me. If I exit the SMS app, or just go one step back to where you see the list of people whom you talked to, then I will get the notification sound that someone just texted me. Weird, huh?