IK refusing to work on specific bones in specific armature

So I’ve got this INCREDIBLY vexing and annoying issue at the moment. I’m wanting to rig up one of my character’s arms with IK so I can have him wield something two-handed. The issue is that the IK is absolutely refusing to work, and I have no idea why.

Instead of following the target bone, they attach to some random point in space and follow that. It only happens with certain bones, as I’ve got perfectly good IK in the legs.

I’ve tried doing it with other characters using a variation of the exact same rig, and they work just fine; it’s just this one rig, and just the certain bones that I need to work with it that refuse to do so.

Any idea what the hell could be going on?

ya gotta post a blend file