iPhone alarms are totally unreliable for waking me up!

My iPhone alarms often don't wake me up in the morning because something is causing them to stop buzzing. I will sometimes wake up long after the alarm went off and I'll glance over at my phone, which indicates that an alarm is going off and shows it's showing me the 'Stop' and 'Snooze' buttons, but it's not making any noise. I've made sure my 'ringer' volume is all the way up.

What causes this to happen and how can I prevent it so that my Phone will reliably wake me up when I need it to?