torque and identity pair

Hi! I'm working on a 3-phase synchronous motor with interior permanent magnets. I have to perform both stationary and time dependent simulations. I split the air gap into two parts connected by an identity pair wih an applied copntinuity in magnetic vector potential. When I try to calculate forces and torques, with the "force calculation" law, on the boundaries connected by the identity pair I always obtain zero (N or Nm) for any position of the rotor!
I've noticed that this happens too with the "2D generator" in the model library.
Do someone know why this happens? And what can I do to overcome the problem??
Thanks
J.