I think I have figured this out and will make some comments about how I think Evernote thought users would approach stack creation, which is different to what I wanted to do.
The instructions for stack creation on a desktop are:
Mac and Windows Desktop
Select the Notebooks from the side navigation menu.
Drag and drop one notebook on to another notebook. A new notebook stack containing both notebooks will be created automatically.
Drag and drop any other notebooks you want to include on to the new notebook stack to add notebooks to it.
What is not stated is that the two notebooks must not be in a stack already. If you drag a notebook that is already in a stack on to another, then nothing happens. To create a new stack you must have two notebooks that are not part of a stack and drag one on top of the other, which will automatically create a new stack.
I guess Evernote’s thinking is as follows. Say you are interested in Golf (I’m not by the way). You create a notebook of notes about different courses. You create some notes about club committee meetings and put those in another notebook. Thinking it would be useful to group these together in a stack called Golf, you can then drag one notebook on top of another to create the stack. A sort of bottom up approach.
But say you decide to take up golf and know that you want a stack which will contain notebooks on different aspects of the game - courses, committee meetings etc. You want to create the stack before you have any notes about it - a top down approach. The only way is to create two ‘dummy’ notebooks and drag one on top of the other. Of course you can rename the notebooks and stack later on.
This last approach is what I was trying to do, and hence my failure to create a stack as I was trying to drag one notebook in stack on to another. Note also if you want to create a new stack with notebooks from an existing stack, the only way is to drag them out of the stack first.
The whole operation seems to be a little clumsy to me and others have commented that it would be much better to have a menu command or a ‘+ new stack’ button, like there is for notebooks.