Hmm, it could be that how to put it. As with Erebus if im using just the vampy bite attack, its bit random how it works. Sometimes when I activate it on a running enemy he simply dosnt execute the command at all. It could be that when I uhh... 'activate' the command for him to attack the enemy was in range, but as game perhaps lags a bit or enemy is moving. He simply oversteps the powers range and suddenly the command is invalid and cancelled.
So I need to use it again, but its somewhat related to whole fog of war problem? When something exceeds your charachters either power or visions reach he simply quits doing anything.
Its not that I personally mind the fog of war or think my minions should be able to autoseek and kill a completely fog covered enemy. Its more like, uhh they dont work as 'intended' If I ask them to attack person on corner of fog of war and my minions aint there allready. Instead of at least moving to the spot I asked them to attack they dont. They stop moving and doing anything, ignoring attack order. It forces generals or even players to just click abilities or commands over and over as on such edges of fog or perhaps range things just start to fail like that.