Tiller Pilot failure - any suggestions

With my previous MWAB, I managed it to self steer! Half a mile of bungy thingy and it worked. Can't remember how now. But I could stand by the mast and alter her course 5 degrees one way or other by taking a step either to port or starboard.
 
Well I don't know. Today, out of the Quarters, it was fine. Turned onto the wind to raise the main - absolutely fine - for quite a while because I bungled putting the main up and so took a while. Then occasionally beating to windward up the Blackwater to get up to Caroline and back - it was fine steering to wind. Turned downwind just under the genny, running with 15 to 20 knots apparent and it didn't like it. I know it doesn't like that being a MWAB. Set to set to course while I went on deck to tidy up the Mainsail and it calmly turned 90 degrees to starboard! It didn't try to get back to the course which suggest summat is up.
 
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