Hello skoppang

Could be any number of things. I recently fixed a 68 DT1 engine with the exact same problem, and it turned out to be ... that whoever was in the engine before me had left out a particular shim on the secondary transmission shaft, that prevented it from going into fifth because the shift fork couldn't complete the shift (throw/movement) without it.

Another suspect area to check would be the shift cam detent mechanism.

Note: Often times the transmission shaft must be rotated a bit to complete a shift, because the cogs on the gears aren't nessesarily lined up.