Alarm not working?

This happens sporadically but I've noticed that every once in a while (it just happened to me and it hasn't happened for the longest time) the alarm on my iPhone will go off internally but it will not make a sound or vibrate. What I mean by this is that if I press the home button (after waking up late because it didn't ring) I would see the alarm screen as if the alarm is playing but there would be no sound or vibration. I've noticed that this only happens to alarms that I set manually on the spot. It does not happen to alarms that I have set to repeat on certain days like every Saturday, Sunday, etc.

I would see the screen in the pic below when I press the home button but there would be no sound or vibration

 

Well so far so good. I've restored from backup and I've been testing the alarm and using it. No problems so far but I mean this happens sporadically. I think this has only been the second time this has happened but, again, this hasn't happened for a long time (I think it's almost been a year since it last happened). Maybe just a glitch thing or something.