OK Grant, this is normal if it is your first one.
It is difficult to understand when reading and without seeing it in action.
After you pull the selector rod all the way out and handlebar well beyond 4th gear position you have to:
engage the pivoting block at the end of the selector box in the space of the selector rod (top end ).
the pivoting block at the end of the selector rod is made of a small pivoting arm and at the end of this arm you have
a small pivoting metal part of about 1cm.
You have to handle the selector box in a way (at a certain angle)that the small pivoting metal part of the pivoting arm slide in the space of the selector rod. (To allign this before sliding it into the space you can use a small screwdriver) this is the part that pull and push the selector rod, and if it is not in its position it won’t work.
After doing it turn the handlebar in 1st helping it with tyre movement.
Try again. Before doing it study well the pivoting parts and where they have to go. This will help.