Help with Thirst Undending!

[hahahahaha]

ok, I'm a blood elf warlock, and i accepted a quest called "thirst unending" it says to use mana tap on 6 creatures with mana, everyone is saying use it on mana wyrms and i am but the counter for the quest isnt counting the times i use mana tap on them. Now don;t give me these stupid answers of telling me how to do the move I KNOW HOW TO DO THE MOVE! >.< but i use it and it doesnt count it then i tried killing it right after using it, didnt help, then i tried using arcane torrent after using mana tap but it just wont count it. is this a bug or am i not going about this in the right way?