The throttle controls the speed governor for the diesel engine, the pedal determines what is essentially continuous gear ratio transmission. If you leave the throttle at the idle setting and try to go forward with the pedal control, it is much like trying to launch a manual transmission car with the engine idling. Pushing on the pedal on the tractor doesn't increase engine speed, it just increases the load on the engine which in your case is set at idle speed by the throttle control.
Unlike a car, you need to set the throttle open to a reasonable governed speed, usually at rated speed when using the PTO and maybe half way between idle and rated while just moving the tractor at moderate speed.
Rodger