DIAGNOSIS The cup is not making flush, even contact with the base drive socket. The plastic alignment ring on the bottom of the cup is damaged or the cup itself is warped.
FIX Inspect the bottom of the problematic cup. Locate the plastic ring with the drive slots. Check for cracks or missing chunks in this ring. Compare it to a cup that does not wobble. A damaged alignment ring cannot be repaired; the cup must be replaced.
IF THAT DIDN'T WORK The motor base drive socket may be damaged. Inspect the raised plastic teeth inside the base socket. If any are chipped or broken, the base will not hold any cup securely.
HEADS UP Never force a cup onto the base if it does not align easily. Forcing it can shear off the drive teeth, damaging both the cup and the base permanently.