4th speed hard to enter

[Fumello]

I have istalled a quaife ATB on my gearbox changing everything is usal to be done when a gearbox is opened: springs balls bolts seals and bearings. The reassembled gearbox matched perfectly but when i try to put the 4th speed from the 3rd that is hard to enter and sometimes i must put neutral to put the 4th. The box have been opened and controlled a second times but apparenlty seems allright. Do u know if it's am usual trouble on the isuzu gearbox and do u know from what it can belong?

Thanks for your help

Francesco