What model and serial number Exmark do you have?
Does this look like your pulley setup?
Check all the pulleys (spindles and idlers) for easy rotation without binding to make sure you don't have a bearing that's seizing up.
Also check the pulley to be sure they are lined up properly. If you have a loose idler pulley, or a bent spindle, the belt will be thrown of because of the different angle.
Check for any springs on the idler arms that may have come loose or are weakened.
Check for proper belt tension and adjust the engagement linkage if necessary.
Hope this helps! Let me know...