Blade stops turning but motor still running

Question

Old machine from my father. Tried making ice cream but after a short while the blade stopped spinning, stalled. The motor and belt are still turning but the frozen layer of ice cream on the inside of the bowl is too much for the blade to cut thru. Is this normal? Is the connection of the blade shaft to the drive pulley designed to "slip" under heavy load? The instructions say to "churn" the mix for about 20 minutes but we only ran about half that before the blade stalled. While there was a thick buildup of frozen [firm] material on the sides of the bowl the material in the middle was still fairly runny.
Would appreciate your comments.