Antimatter menu - dropdown dowsn't work from all pages

I thought I’d post this now as it may be linked with my previous post.
I have the Antimatter theme with very little in it. I have set up a few subpages and a subsubpage.

When I am on the homepage, the dropdown menu works for both of the pages underneath Home, (ie, pages A & B); subpages under A (Aa and Ab) and the subsubpage under Aa (Aa1).

However, when I move to page B, the dropdown menu doesn’t show under Page A on the menu bar. Similar issues happen when I am on subpage Ab.

Probably something really basic, but it’s eluding me so far…

Many thanks again, Luke

The breadcrumb issue may be tied up with this - I don’t know.