There have been many times where my skills are available (I mainly play UB and I'm talking about poison vomit or foul grasp), and when trying to use my abilities on an enemy, it doesn't work.  It won't.  Yes, I have mana.  Yes, I'm the right distance, I'll click the hotkey, click (WHILE HOLDING CTRL SO I DON'T TARGET ANY OTHER DEMIGODS BUT THAT ONE) and the ability won't be used.  It's really annoying.  Same happens when I try to use the parasite egg trinket on other demigods-- it takes 4 or 5 tries to use it succesfully and you can't afford to do that with the other team slaying you while you stand still.  It's ridiculous.