DIAGNOSIS The plastic threads on the cup or the blade assembly are cross-threaded or worn out. This is a common wear item.
FIX Check both parts. Look for damaged, flattened, or shiny plastic threads. The problem is usually on the cup. You must replace the damaged part. Cups and blade assemblies are sold separately. Order the correct size for your Pro Plus model.
IF THAT DIDN'T WORK If a new cup and blade still won't thread, the blade drive socket inside the power base could be damaged. Inspect for cracks or debris. Base damage means base replacement.
HEADS UP Always start the blade by hand. Do not force it or use the motor to tighten it. Hand-tighten only until snug. Overtightening causes this.