I was playing around with this skill and it seems the actual trigger area is clearly smaller than the graphic effect on the ground. Several times i had enemies walking straight through the mines, or standing almost halfway on a mine. It looked as if they were partly touching them, so it should have triggered. It's a very very high cost skill so you would expect it to trigger more often. I don't know weather this is a bug or intended. Either way graphics effect need to match the actual trigger area more precisely. Preferably increase the trigger area slightly, as it's too easy to slip through.