Its possible the dial itself thats part of a pcb isnt fitted centrally within the plastic housing, its best to remove the knob & check the alignment behind to see if its central, you can also compare the gapping all round the base of the dial in 1st instance to see if its equal, if not then thats your issue, although if its getting tightly stuck, could also be the pcb dial itself, but atm I'm suggesting check the plastic knob fitting etc 1st.