I think that I'm having more trouble when shifting between 2nd and > 3rd gears than any other combination (although 2nd to 1st is problematic, too). This leaves me contemplating the gear selector forks inside the transmission.
The reason I replaced my shift forks, was that it popped out of gear when bench testing after shimming. The faces were wore enough so it did not push'em far enough to "lock" in place. With the drum flopping around forward or aft before shimming, it "moved" where it needed to, to allow engagement.