Trolls only do the ranged stone move if the target is to far away, can't be reached or if the target is standing directly in front of the player. The pathfinder probably gets confused and thinks the target is unreachable if the target is to close, otherwise the stone move works perfectly fine.

This video shows that the stone move only is broken when the target stands still and allows for the troll to get to close.

http://www.youtube.com/watch?v=d7PppghgkwI

Maybe a possible fix would be to have the troll check if the target is within its attack distance before doing the move. Something like preventing stone move if its target is inside a rectangle around the troll.