The rule is it has to be on the same foothold as you, to limit it's power and stop issues like dragging those 2 mobs off the edge or not all the way towards you. It's purposeful to limit the maps it can work on and not make it too OP.

Thanks for the detailed screenshot, Tai. As Mark said above, this one is on purpose: the Monster Magnet only pulls monsters standing on the same foothold as you. That keeps it from dragging monsters off ledges or only half way across, and keeps it from being too strong on maps with lots of stacked platforms. So I'm closing this as working as intended.