DIAGNOSIS: The base isn't detecting the cup as properly locked in place. This is almost always because the cup's blade assembly isn't twisted all the way clockwise onto the base.
FIX: Take the cup off. Look at the black plastic ring on the bottom of the blade assembly—it has two arrow markers. Align the arrow on the ring with the unlock symbol on the cup. Press down and twist the blade assembly clockwise until the arrow clicks firmly into the lock symbol. You should feel a solid stop. Now place the cup back on the base and press down slightly while you turn it clockwise to lock. The green light should go solid.
IF THAT DIDN'T WORK: Check for debris—a tiny piece of food or a dried drip of smoothie on the black ring or the silver drive socket on the base can block the connection. Wipe both clean with a damp cloth.
HEADS UP: The motor won't engage if the cup isn't fully locked; it's a safety feature. If you force it by pressing hard while twisting, you can wear down the plastic tabs. A gentle, firm turn is all it needs.