Stuns will fail for three reasons:

#1 The mob cannot be interrupted
#2 You missed!
#3 and the most likely: Stuns are effected by diminishing returns. Meaning your first stun will have 100% duration, the 2nd will have 50% duration, the 3rd will have 25%, and the 4th will do nothing. I believe its a 30 second time frame... might be 60 seconds though, I'm not 100% sure off the top of my head.