Target Abilities sometimes don't get triggered, but animation runs (e.g. Magic Missiles)

[Kralith]

In several days of fighting i tried to figure out, what causes that abilities for that you need a target sometimes don't get triggered.
Especially with Magic Missiles i often have the problem, but not exclusive.

Mouse is on target, the target attack symbol is appearing, the target is outlined, i am not standing out of sight.
But the Ability has difficulties to get triggered, i need to hit the Action key many times, before something happens.
And sometimes the animation and the sound of the Missiles are playing, but you don't see an effect and after the animation ends, the ability don't get a cooldown, because it was never performed.
Yes i know, there is a visual bug, where the cooldown is sometimes not shown correctly on action bar. But my ability wasn't at cooldown at all, because when i tested it consciously, the missiles was my first action for the fight and not used in the last 30 seconds.

I wonder, why i do not see another report of that, or the search just don't give me the right reports.
@TrueCrimsonFTW at least mentioned it here.