I had this similar problem. Remove the shifter, then remove the clutch case/front sprocket case. Push and pull the shifter rod. If there is play, then that might contribute to you not being able to shift up. There is a small circlip on the shifter rod that holds the rod in place. If there is play, pull the shifter rod all the way out and hold it, then push the small circlip all the way until it touches the washers. Make sure there isn't any play on the shifter rod, then reinstall everything.
Hopefully that helps!