DIAGNOSIS: The plastic threads on either the cup or the blade assembly ring are damaged or have food debris jammed in them.
FIX: Inspect the threads in good light. On the cup, look inside the neck. On the blade assembly, look at the large black ring. Use a toothpick or a stiff brush to dig out any packed-in seeds, spinach fibers, or dried gunk from the grooves of the threads. Wash both parts thoroughly. Before screwing together, align the threads perfectly straight and start turning counter-clockwise until you feel a slight click/drop, then turn clockwise. This ensures proper thread alignment.
IF THAT DIDN'T WORK: If the threads are visibly stripped or cracked (look for white stress marks in the plastic), the part is damaged. Forcing it will ruin the seal. You need a new cup or blade assembly.
HEADS UP: Never overtighten. The seal is made by the rubber gasket, not by how hard you wrench the blade on. Hand-tight is sufficient. Overtightening is the #1 cause of stripped threads.