If you look under the belt cover and engage the auger handle the belt engages and spins, the pulley spins on the shaft of the impella but the impella does not turn. If you spin the impella from the outside it moves freely but the pulley does not move. The shear pins are OK.

First there is a set screw that holds the pulley to the shaft. If it is loose or stripped it will not engage properly. Next, there is a set of gears between the drive shaft and the auger, that's right in the middle of the auger. This gear box changes the rotation of the drive shaft 90 degrees. One of these gears could be bad. As a result that shaft will spin and the auger will too. It takes a little doing to get the box apart, but with simple tools and patience you can do it, and get replacements at your dealer or online. Usually the shear pins fail first. On last thing: a worn belt or bad tensioner, or tensioner spring will allow slippage and not engage the impeller fully.