DIAGNOSIS The locking lugs on the pitcher base are not aligning with the slots on the motor base. Often, the pitcher is mis-threaded onto the blade assembly.
FIX Separate the pitcher from the blade assembly. Align the arrow on the blade assembly with the unlock symbol on the pitcher. Twist clockwise until it clicks and the arrow aligns with the lock symbol. Now place the pitcher on the base and turn to lock.
IF THAT DIDN'T WORK Inspect the plastic locking lugs on the bottom of the pitcher for damage or breakage. A broken lug requires a new pitcher.
HEADS UP Never force the pitcher onto the base. Misalignment can crack the pitcher or damage the motor base sensor.