I have a label 'alpha' and under that I have nested 'abc' and 'def'.
When I tag an email with label "alpha/abc" and I open label "alpha", why don't I see that email?
I only see the emails tagged with "alpha" and none of the messages tagged with the child label.
Best Answer
You will need to add the parent label as well, even if the sub-label is supposed to be nesting.
When you look at the list of labels, you will see the parent and sub-label (or child label) as separate choices.
This is why when you click on a parent label it will only show emails tagged with that. It will not include messages "under" that tag.
That hierarchy is mostly for visual organising instead of applying to all child messages.