DIAGNOSIS The blades are spinning freely without engaging the contents, usually because the ingredients aren't being pulled down into the vortex. Yeah, this one's common.
FIX You need the right liquid-to-solid ratio. For these small cups, start with your liquid (milk, juice, water) in first, then add softer ingredients (yogurt, banana), and put the hardest/frozen items on top. That creates a vortex that pulls everything down. If it's just whirling, stop, take the cup off, stir it with a spoon to break the air pocket, and try again.
IF THAT DIDN'T WORK Check if the plastic drive coupler on the bottom of the blade assembly is worn down or cracked. If it's slipping inside the motor base, the blades won't spin with enough torque. You'd need to replace the blade assembly.
HEADS UP Don't overfill the cup. The max fill line is there for a reason. If you go past it, there's no room for the ingredients to move and blend, they just get stuck.