Need some advice on what to look for on a 2014 Evo 2t 300 that will occasionally fail to up-shift into 4th gear (remains in 3rd) and at other times will fail to up-shift out of 1st gear.  In both instances the problem will correct itself after repeated shift attempts and then will shift normally. The selector gear rivet has been checked and is secure in the selector gear mechanism.

Only things I can think of is a worn or damaged shift drum or a damaged shift fork. A damaged gear might do it but would probably make quite a bit of noise.

Is it just those two gears? Is the shift from fourth to third OK?