I have an “Esper Sentinel” out with a counter on it. My opponent cast a “sheltered by the ghosts” to target another one of my creatures. Usually, my opponent would have to choose not to pay the cost for me to draw 1 card—cool. But he didn't pay for it, and the effect didn't go off. I didn't get to draw my card for it, and he got to cast his spell normally.

This has happened a couple of times already (three or four). I thought I wasn't paying attention to the fact that it can only trigger once per turn, but I have been paying closer attention to it, and it keeps happening…. Help.