Check pulley alignment. You could have a problem with the pulleys not lining up, And the belt tension arm will get worn over time, and sag, which throws the belt off. Also check the condition of the belt guides. If they are not correctly installed with the correct spacing from the required pulleys will allow the belt to come off when the belt is disengaged on a manual pto unit.
I have also noticed that you have been replying to post the are 4-5 years old. Check the date in the upper left corner before replying. Reopening old threads and multiple posting tends to irritate the techs around here.