DIAGNOSIS The adjustable shoe is jammed, almost certainly by a buildup of material—like melted plastic or compacted sawdust—in the guide channels. knob just loosens the clamping force; it doesn't free a physically stuck shoe.
FIX You need to break the bond. First, loosen the shoe adjustment knob completely. Don't force it. Get a putty knife or a thin, flat piece of metal. Gently tap it into the gap between the shoe and the main saw body, right along the slide rails. Work your way around to try and pry the shoe loose. Once you get some movement, spray a penetrating lubricant (like WD-40) into the channels. Work the shoe back and forth vigorously until it moves freely. Then clean out all the gunk and old lubricant with a brush and rag.
IF THAT DIDN'T WORK If it's truly seized, the slide rails on the shoe itself might be bent from an impact. You'll have to remove the shoe (by taking out the pivot pin once it's loose enough) to inspect. A bent shoe usually means replacement.
HEADS UP After you free it, make a habit of blowing out the shoe mechanism with air after heavy use. A little dry lubricant (graphite powder is good) on the rails will prevent this from happening again, unlike oil which attracts more gunk.