Yes.
There was a post earlier this week asking to "name this transmission part" had this exact issue. You could have installed a shift rail spacer that does eactly that. And there are any number of other possibilities.
-The 5th gear slider installed upside down is a common one.
Pull the end cap and take a look at 5th and see what you see. This can be done in the car. If you see nothing obvious, with the end cap off, remove the 5th gear shift fork and slider. Then see if you can move the shifter into what would be the 5th gear position. If it goes, you know the problem is with the slider/synchro/fork assembly. If it doesn't, then the problem is in the linkage.
If the problem is in the slider/synchro/fork assembly, you can probably fix it in the car.