There are a number of possible reasons this occurs.
- Slips Out of Second
- Bent Shifting Fork
- Worn Gear
- Weak Poppet Spring
- Interlock Plunger Not in Place
This comes up so often I figured I would post the “book” answer. I hope this helps.
Interested in WW2 jeeps? How about the history of the early jeeps that was actually written during WW2? Jeep – Its development and procurement under the Quartermaster Corps, 1940-1942