Massey ferguson 4255 gearbox

[6480]

Hello everyone
I have a mf 4255 with a 12x12 manual gearbox
Lately it has started to slip out of reverse on the left hand shuttle lever
If the tractor is reversing back with a wee force on it its fine
If I am reversing and I let the revs off it slips out on the shuttle lever
It fine going forward
Has anyone any ideas what the problem is

 

[Nearly]

@masseybreaker

 

[dee]

Check the top of the gearbox that nothing is stopping the shuttle moving fully into reverse.

The shuttle lever is attached to the back of the engine with a bracket.
There is a plastic ball inside the bracket make sure it's not worn or has fallen out