Are all roller chains present? What condition is the rubber on the belt drive rollers?? If tailgate is remaining closed then I suggest to check for correct belt routing?
Several yrs back My neighbor who had a newly purchased model 457 asked why the belts wouldn't turn when pto was engaged. He stated he'd R&R a tension roller to R&R brgs. I stated he had installed roller correctly therefore belts were routed incorrectly. He stated no he had reinstalled roller by viewing a photo he'd taken. I told him to get the photo & sure enough I pointed out to him he'd installed roller incorrectly.