350Z - 5th & 6th gears

[LordGrover]

Since my clutch and flywheel were replaced last year, fifth & sixth have been a little notchy or difficult to select. Yesterday, I couldn't select either. I'd assumed there is an H-plate or guide on the top of the box which is misaligned but I now think that's not right. After a bit of experimentation, I discovered pressing the stick a la reverse gear allows easy selection of both fifth & sixth.
Any ideas peeps?
Is it safe to pop it into local Nissan dealer and not get screwed over?