war trolls - orc tower

[obregon]

When my warlock uses binding blast on trolls in orc tower, they simply ignore the binding effect. Their save rolls show, fail miserably, and they keep on coming? There is no other message, such as for immunity to mental effects, that appears on every other monster that is not affected by it such as golems, mindless undead.

I'm not aware of any 'in game' reason why a war troll should be immune to stun, is there something I've not thought of?