Drive Spindle or do I mean the spindle drive....
Can you provide model # (eg 924050, BUT not all ST824 are 924050 ) AND serial number.
I suspect the Drive Spindle. I just had the same problem in my ST824, and replacing the rubber wheel and adjusting the control-rod did not help.
Drive spindle sits behind the tractor (engine) pulley. on the same axis as the aliumimum friction plate. NOT the rubber wheel, and not the ' other spindle' that moves the rubber wheel on the hex shaft, controlled by the speed selector. Please refer elsewhere to discussions related to the rubber wheel/friction plate/speed selector. There are lots of such references, but not too many regarding what I call 'Tractor Drive Spindle' that I describe here:
If you post the model number/ serial number, then I can more accurately point you to what I'm talking about. BUT the naming used on some Parts Lists is confusing, and the exploded diagrams have also changed , so the orientation for the amateur (like me) can be tough.
Remove the belt cover, depress the Traction Drive (left) handle. and look down from above. Cycle the handle and look for the large tractor pulley to move about 3/16 of an inch to the rear, so as to make the Aluminum drive plate contact the rubber wheel. If there is little to no movement you will have to continue with 'separating' the tractor(engine) from the blower(auger/bucket) attachment, and remove the tractor pulley (use the small hole in the pulley for leverage; the shaft nut requires a 1" wrench !). Then you have direct access to the Clutch Fork, Drive Spindle, Drive Spindle Housing, and the two springs that return/retract the movement of the Traction Drive Handle Control Rod. Removing the engine idler pulley (and belt finger) will help with removal and installation of the engine/tractor drive belt. The pulley shaft is keyed, so its lots of fun to recenter it back on the freely spinning spindle shaft...
The Clutch Fork moves the 'Roller' on the Drive Spindle. The Clutch Fork mounting/pivots could be bound up, or the Drive Spindle is not sliding in its housing. For reference the housing is bolted to the inner plate of the Engine Frame (3 bolts). Some other brands use a 'swing plate', but not on the Ariens 924050.
I sprayed the drive spindle and Clutch Fork pivots/sliders with PB Blaster, then the next day I made sure all had good travel. Cleaned what I could, rotated the Spindle in the housing ('Roller' removed), then applied 3in1 to the drive spindle and sprayed lithium grease to the clutch pivots. Grabs like crazy now! Could have spent more time cleaning/smoothing up the clutch fork, and I'm not certain which part was binding: spindle or clutch fork assy, but I was just too glad that was finally moving... I will follow-up with some images.