Why doesn't apple watch silent, haptic alarm always work?

[iMore Question]

I wear my apple watch to bed. I use the silent haptic alarm to wake me every morning. I continue to use a back up alarm clock. Some times, I wake to the back up alarm clock and when I look at my watch, the alarm is on the screen (with the options to snooze or dismiss), but there are no haptic touches. Any thoughts would be appreciated.