4th gear sometimes difficult to engage

[Graham O]

I rebuilt the gearbox and I find that at times 4th gear is hard to engage.  If I release the clutch and then try again it is the same.  The problem seems to be a random one, sometimes all is smooth, other times it balks and I almost need to force it into gear.  Any suggestions please.
I do need to attend to the gearbox at some stage as I did not replace the selector springs and balls and it immediately jumps out of 3rd gear on the overrun.
Graham