L 300 Switching problems

Hello together, have recently problems with my L300 bus 2.5l Built. 1993 (Lenkradscheitung!!!!) Sometimes I get the first and second gear almost not switched correctly, because the shift lever simply does not want to go further. (hacked) The problem only hit with running engine and can occur immediately after start or even much later. If I then switch and can move the shift lever to the right position, it also works without “cracking”. Someone a Idea? Clutch disc, transmission oil? Thanks for the Info’s Mountie72