Although I’ve added 16 categories only 14 are showing in the top main menu. I have tried deleting and adding the missing 2 again but still they are missing? Why is this? Thanks.
Please make sure that the block that you are using to display category menu, has Categories type, not the Menu one.
The block with the Menu type is being filled manually:
So you you need to add the new categories to the menu manually.
Whilst the block with the Categories type is being filled automatically.
I hope it will help you.