1. Shift linkage out of adjustment.
2. Shift linkage bent or damaged.
3. Shift linkage parts loose.
4. Shift levers, shafts or forks worn
5. Shift cover loose or gasket damaged.
6: Transmission misaligned or loose.
7. Synchronizers worn or broken.
8. Clutch gear bearing retainer broken.
9. Clutch gear bearing worn or damaged.
10. Clutch pilot bearing worn or broken.
11. Mainshaft and/or pilot worn or damaged.
12. Mainshaft bearing worn or damaged.
Hmmm. Maybe a visit to the gearbox reconditioners is required. That said, i dont want to worry you too much eek! You could check the first few items yourself as the shift linkage may well be worn and easy to spot.