My friend and I noticed it when the game was released and I reported it months ago in this forum. Each patch we've been expecting the problem to be fixed but so far it hasn't. We haven't been able to test it with the latest patch yet.
Like Backspace1k mentioned it only occurs during multiplayer play. We noticed it happens when you use a skill too close after using another skill. If you pause a second in between skills you never have a problem.The problem occured more often at release and has gotten better each patch, but was still there in patch 1.0.15.0.Both my friend and I have the problem no matter who is the leader.
So for example if jump in using Wings of Ishmir and immediately cast Bladestorm you will get the animation and the sound of Bladestorm but the monsters take no damage (health bars do not decrease) even though you are spinning right on top of them.
Another example is your Bladestorming and you stop to immeadiately cast Juggernaut then cast Juggernaut again to explode it. What happens is you stop Bladestorming, you hear the Juggernaut sound and see the animation (shields surrounding you), Juggernaut does not explode, it just goes away and does no damage to any of the monsters in range.
I agree with Backspace1k about the problem most likely being to syncing issues between the client and the server. It's like the client controls the animations but the server controls the damage. The client knows that you've finished casting the 1st ability when you cast the 2nd so it shows the animation for the 2nd but the server thinks your still casting the 1st ability when you go to cast the 2nd ability.