here's a thought. Maybe you DO wake up, but don't remember it?

This happens to me a lot. I'll set my alarm for 4:30AM and wake up at 7:00 AM, But i'll look at my clock, and the alarm is shut off. Either I woke up to it and shut it off without remembering it, or someone else did. No one else would shut it off here though, so i know I had forgotten.

If it IS you forgetting, try to set your alarm up at sporadic times. Always different. Try not to make a pattern or else your subconcious will recognize it as that and it will write it off and you'll forget.

This is all just a theory though.