Ok I have had an iPhone since 2007 with the original and every iPhone I have had since has exhibited this same problem. I will set the alarm on my iPhone and yes I go back to check and make sure that is was either am or pm, and it has always been set right but it just decided to not go off. I have narrowed it down to I believe it happens when I set it after midnight, for early morning, within about 6 hours of when I set it. It just won't go off. Now it isn't predictable so I can't make it do it. But since my first iPhone in 2007 I have had this problem happen to me maybe 20 times. And of course it has made me late to class and now WORK! I have to set a back-up alarm now to prevent from not getting up on time. Anyone else had their iPhone just not go off when set correctly?