Do you mean the unit jumps out of gear?
Cable assembly may be bent or damaged
check to make sure that the cables or linkages move freely and do not bind.
Check the gear shifter to make sure that there is no interference with it and its boot on the floor or at the transmission
a misaligned or loose transmission case and /or clutch housing may also be the cause
check for bent or worn shift forks, levers, or shaft, worn input shaft pilot bearing, worn dog teeth on a gear or synchronizer.