Check belt alignment and tensioner and belt condition , also debris on the pulley surface .

There has been a bulletin or two on these tanks. Post the model from the chassis?

Model number 53AB5GFZ750

I didn't find a bulletin for this, just one for the bagger. So this is a tough one to diagnose over the internet. A good place to start is which pulley that the belts seems to launch from. Off the driven deck pulley or a mule pulley? It would be good to know that each component on the deck is working freely by removing the secondary belt and noting roughness in the idler or spindle bearings.

Absolutely as Tom has advised bad bearings are the most prevalent cause of pulley misalignment , which will quickly cause belt displacement .

Plus this is one of those lovely mule pulley set ups. I just cringe when someone drops a Cub mule pulley deck problem on my lap. Bad bearings set up the belt for extra drag. When you chase something like this, you look at it from the belts perspective of PUSHING its way around through the pulleys.

Thanks guys for all the help, I took the belts off and found one of the pulleys at the pto was locking up, installed new pulley now working perfectly Thanks

Good to hear you have identified the problem !