Need Help! Can't Jog axis 4

Please help, I'm stumped. I'm trying to add a 4th axis to my mill. Everything appears to be hooked up correctly, the servo motor is humming and ready to go. I cannot seem to get the 4th axis to move though. When I press the jog buttons on the mach 3 screen, the axis does not do anything and the DRO in mach does not move. The axis is enabled and all the settings appear to be correct. The other axis work fine and jog as they should. I'm hoping I'm just missing a simple setting, but can't for the life of me figure it out. Any help would be greatly appreciated.

thanks,

Brian