I would say the shifter is more than likely related to your 5th gear issue. If you still have it, go back to your stock shifter and see if you can get into 5th without issue.
There is really no adjustment on those short shifters so unless a bolt has come loose or it was installed incorrectly you can't do much. Like Zbullet said you should check you didn't overtorque the pivot bolt/nut, it should not be too tight or else the shifter is hard to move.