I think some alarm clocks, especially recently, just don't always ring when they're supposed to. There are some mornings where I've been wide awake and noticed that it didn't go off at the right time. In my case, my clock has a feature where if you hit the conveniently located button that stops it from beeping when it's not actually beeping, it starts making the planets move faster and I think it doesn't beep while that's going on. Maybe I accidentally double-tap the button some mornings. As for your clock, perhaps it has a similar mode. Or maybe you flipped the switch through the tape.