Is the belt loose? Maybe you need to replace the belt. Check the throw arm for the idler pulley if it's a manual engagement. Also, make the sure you belt and all pullies are clean. If they have grease or anything else on them, the belt might slip off.
I dought it's to tight, usually if the belt is tensioned right the lever will sort of pop into place real snuge, i don't know exactly how to explian it but if the belts loose the lever won't tighten up correctly and it will wander back. Thats what i think it probly is. Or one of the linkages could be bent.