I'm creating the skeleton for a character I have made in Maya, but am having trouble getting IK to work on his arms.

I made a skeleton for a model the other day and had no trouble at all. All of the limits and degrees of freedom are the same on this skeleton as on my previous model (and the skeleton itself is almost identical), but for some reason this arm refuses to bend. Every joint in the arm won't rotate when I apply IK to it, it just moves around dead straight.

Any ideas what's wrong? His legs are working fine, so I don't see what's different.