Sounds like the selector has slipped off the geartrain its a funny thing that moves both top centr gears and bottom centre gears (2nd and 3rd gear only) you can suss it out with all the gears in the left case
The neat thing is that each gear selects in turn along the shaft from left to right
After the cases are together in top gear I adjust the selector rod (directly below the clucth) screwing clockwise until third and fourth are just touching then back off about 1/4 turn the gears should work ok then
Remember the clutch centre locknut should be tightened as it holds the top shaft in position
Hope this helps