The knobs have a small piece of metal in them that aligns with, and grips, a flat spot on the shaft that they control. Quite often this piece of metal will work loose and cause the problem you are having.
There is a simple remidy to this problem.
Contact the appliance center you purchased the unit from, and if they sell parts, order a new knob for your unit. They are not too expensive at all. If they do not sell parts (ie: you purchased it at Home Depot or Lowes) then just let me know and Ill locate you one on line.
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Best regards; The Home Smithy