Pop the clutch cover off and take a look at the shift star on the end of the drum. Some of them are welded and some have a small bolt that holds them on. If it has a bolt, make sure it's tight. Another thing to check is the stopper lever and spring. This is the part that rides on the star. Make sure the roller is tight, not sloppy, and the spring isn't broken. Just another thought, did your shift shaft take a recent hit and get bent? This could make it bind in the left side case but it would have to be a pretty bad bend. These would be the easy fixes. Other problems could be a bad bearing on the shift drum, bent or worn shift forks, galled fork rods, broken transmission clips or a bad shift drum. All of these problem will require splitting the cases to repair.