Question about Tenacity and definition

I appreciate the response Scuba and maybe it is a bug but there have been plenty of times I will stun a 5's on the other team and it doesn't last at all, I will watch him get stunned and the very next move he's already unstunned, the reason I notice this a lot is since he's such a pain on defense and his game mechanics defy logic...lol.. if I get him stunned I'll center attack on him since he can't counter and that team doesn't have Barris which was also one of the reasons I had asked the initial question since I had thought there may be other factors involved in stun duration that I was missing...