I have been leveling the Archer and have noticed that after casting a instant ability that sometimes you cant cast another ability instantly afterwards, for example, casting venemous bite then trying to cast Raging Strikes or Bloodletter which aren't on the global cooldown, they just simply dont cast, if the abilities are instant then they are instant, there shouldnt be a invisible delay that simply fails to cast another ability for any reason.

It is rather annoying because if you fail to notice that they just didnt work you could fail to get a life saving buff, self heal or last hit that finishes the mob.