RobotStudio event

External Axis Wizard

I am doing a simulation for a spot welding gun.  There are two different guns, one is a linear gun and the other is a scissor like gun with a rotational axis.  I successfully made the linear gun into a mechanism and then added it as an external axis with the External Axis Wizard.  However, when I try to do the same thing with the scissor gun, it doesn't recognize it as a mechanism.  Does anyone know why this might be?  The only difference is the joint type.