The dial should change the max speed. It of course does not alter the foot pedal action. If there is no noticeable change in speed with the potentiometer, then there is an electrical circuit problem. (Start with the dial at high speed while the machine is running, pedal to the metal, with no thread in the needle, and turn it down. You should see/hear the difference. If you are running the motor only, you may not see/hear the difference. Might need a tachometer to see. Also speed will likely vary inversely to the load, so make sure there is some load on it.)
Did you buy from a dealer or privately? If you bought from a dealer, go back to him for support.
If a private sale, still need to go back to the seller and negotiate a fix or replacement. Else you will have to find an actual circuit diagram and trouble shoot yourself, or replace it.
Tom