I was thinking something like tyres wearing differently front to back that had caused a gradual difference between front and rear ration.. causing binding. Try slipping it in and then straight out again without moving. If it comes out then , then it is binding up. Or, try slipping it out while the bikes moving, and in particular try blipping the throttle just as you go to slip it out so the load comes off the gear. Also perhaps try driving straight backwards without turning and then again turning the steering to see if that makes a difference.. If any of those things makes a difference then it's probably a binding up between front and rear putting a load on the gears.
Other than that, some part of the linkage going over center ? So it has good leverage pushing into gear but then goes too far so the lever is at a steep angle.. But that may not be an option in the case of a yammy..