Polaris 360 wheels not moving however the tail whips

I completely rebuilt my Polaris 360 a couple of months ago. It never worked better in years. Then it just stopped moving/rolling. I replaced the backup valve as it was 12 years old. It did not fix it.

It rolls by itself, no obstruction. Looking into the open unit, all belts are connected and move manually. All drivetrain gears are good. No broken teeth.

The float head is not full of water. The quick disconnect filter screen is clean. If I blow into the feed pipe, air comes out of the thrust jet. All the water management hoses are connected by original zip ties.

Any ideas?