I posted this on steams forums but figured I'll do it here as well. The issue with Concussive Trauma right now is it only applys 1 stack on the initial key press if you hold it down to chain attacks or channel the attack it flat out will not apply any more stacks unless you let go of the button then click again.

Tested with melee and range across all 3 classes just to confirm and yes it does this across all 3 classes.