I have a kitchenaid washing machine and the bottom portion of the agitator only turns briskly in one direction in the normal cycle. The agitator won't wig wag and it makes aloud thumping noise.The clothes get all wrapped up tight around the agitator as a result.The only cycle that it agitates properly is in the delicate cycle.
Is it the transmission, I've changed the clutch and dogs a while ago.
What could it be.

Possibly transmission but @17 years it's time for a new one. You'll spend more time and money and still have a 17 yr. old washer.