Prime for Stun, need an explanation

Could someone tell me why primed for stun works better with the less powerful of the two?

I use Leap Slam, and the most powerful weapon almost never causes primed for stun.

Thanks!

I believe that's because you're "overstunning" the enemies, basically bypassing Primed for Stun.

IMHO that is horrible, we are basically getting punished for building up stun stats and makes us use less powerful weapons. It completely ruined my Stun Boneshatter build.

I've seen a lot of posts complaining, hopefully they fix it soon.

Thanks for your reply, that was indeed the reason, and it's very stupid.
I had to give up my BoneShatter build too.