Oddly, if I understand OP’s configuration correctly it’s not out of whack…at first blush.
Let’s check the motor data on the CIM to see what kind of torque it’s kicking out at 21A. 0.36 Nm is 0.27 foot-pounds, which at 100:1 is 27 foot-pounds at output…which is well within the limits of a 100:1 HD CIM Sport (140 foot-pounds).
If the gearbox is getting stalled, a CIM can kick out significantly more torque than a 100:1 CIM Sport can handle. I would check for binding, go with smaller pulleys to reduce the torque needs, shave weight where it’s possible, and verify any ratchet mechanism is getting engaged in the right direction.