when selecting gear, the normal process should be:
- clutch fully disconnects engine from the input shaft,
- gearstick linkage system moves selector rod, overcoming the ball/spring holder,
- rod moves selector fork & synchro hub towards the gear cog,
- synchro matches the synch-hub and cog speed,
- once the two cogs match and splines align, the hub teeth is allowed to slot/engage fully into the gear cogs female spine. the ball/spring mechanism holds the selector rod in the forward/neutral/backward position
- clutch released,
- torque transmitted through the selected gear jams/locks the synchro-hub against the cogs female spline preventing it from slipping out
so first I'd check if the clutch is fully disengaging.
a dragging clutch will make selection harder to fully engage. if the gear is only partially inserted into the cog it'll likely slip out.
bite point should be bout 1/4 - 1/2 way up. adjust the thumbscrew at the end of the clutch cable under the battery tray to alter the bite point.
next is check the gear linkages for loose nuts, worn slack bushes.
slack linkages may not allow the gearstick to move the selector rod far enough to engage & lock into gear so it slips back.
select a gear and apply light force back into neutral to see if the selector rods ball/spring is holding/locking it in place. if it slips back into neutral with ease, the ball/spring latch maybe stuck,broke.