Does the gear shifter stay in 5th position or does it jump straight out? Could be an issue with the interlock or detent?
Have you physically tried to shift the syncro ring to make sure it engages properly? If its too tight to slide it could be your problem. If the syncro isnt sliding far enough the synchro sleeve wont engage properly with the dog teeth.
There're a whole lot of checks you can do but its really just a process of elimination if you cant find any obvious signs of wear or oil contamination.
Let us know how you go