I'm thinking (hoping for Yang Yang) there may be something binding on the detent wheel, like a piece of spring.
@Yang Yang Pulling the front cover and clutch will expose the detent wheel and spring which will quickly tell you whether you're going to need to go deeper into the cases or not. I think
@warp9.9 has a guide somewhere you can search on how to replace the detent spring, which will have the guide to removing the front cover. In short, it's pull radiator, pull front cover, pull clutch. Not particularly hard or expensive.