Diagnosis
The drive coupler teeth are stripped. The plastic coupler that connects the motor shaft to the blade assembly has worn down.
How to Fix It
Unplug the blender. Remove the jar and blade assembly. Look into the base where the blade post goes. You'll see a black plastic piece with teeth. That's the coupler. If the teeth are rounded or broken, replace it. Part number is 61094 for the Wave Crusher. It snaps in.
If That Didn't Work
The blade assembly bearing might have seized, dragging on the coupler. Check if the blade post spins freely by hand. If not, replace the entire blade assembly (part 61095).
Heads Up
Never run it with a stripped coupler. It will overheat the motor and cause permanent damage.