No 5th gear!

[gtivrt]

All my other gears work and it shifts smooth it just wont go into 5th it just clicks but not real loud just like the gears are hitting on the sides but not engaging. This is just after a complete rebuid and we were only riding to break it in plus 5th gear has the least amount of stress on it (right?). We replaced 3rd gear just to be safe but it looked pretty good. I do not know how it was ridden because I purchased it with a blown motor so perhaps the clutch wasnt used. Any help would be greatly appreciated! I have done some research and I am leaning towards a shift fork but I want other opinions. Do I have to split the case again to replace the fork if thats what it is?

Thank You