Cooktop dual element only runs on HIGH heat

I’m new to the forum but have searched many topics here. Thanks for the expertise and advice.

Our Jenn-Air glass top electric downdraft cooktop has one dual element that will only function on high. All of the other three elements function normally. When we set it to the small element, just the inner element comes on, but at full power. Same with the large element - both parts come on but only at full power.
Is this just an individual control (rheostat) problem? If so, how does one repair or replace the bad control?

Thanks again,

Dave