Dexta jumps out of gear

[gemmachine]

My 1959 Dexta jumps out of high gear when going down hill.
I have unbolted the gear sticks and found three pins that I think are spring loaded to hold the gears in place.
The one nearest the High low gear stick doesn't spring up like the other two which I think it should?
I have pulled it out and oiled it so it is free moving
Pleas can anyone help with a way of fixing this.
Thanks

[Brian]

Usually when Dextas jump out of gear on over run it is bearings or wear on the gears themselves rather than the selector rail locks. think you need to investigate deeper into the box.