The 904 won't upshift to 3rd gear.

If the KD is correctly set to the carb position and the fluid is clean and correct then the options are: leaking seals, slipping clutch pack, loose nut behind the wheel. (Sorry had to throw that one in there lol!) or is the shift lever adjusted properly or does it have a lot of slop and finally just not putting in the proper rooster comb. I’d pull the pan and look for debris, and verify linkage before you pull it. I also recommend using a pressure gauge and check for pressures to verify operation.
Just my humble opinion-
Syleng1