After creating a new Primary Menu, the menu dropdown is not working on mobile


We had a Primary Menu called NewMain. I did a major edit of the menu and must have reached the maximum number of menu items because it became corrupt. So, I created a new Primary Menu called NewNewMain. It works fine in Explorer, Chrome and Firefox on my computer. However, there used to be a 3-horizontal-bar icon on my mobile phone which you clicked on to open a dropdown menu (all menu items listed one under the other). Now it just says 'Menu' and when you click on it nothing happens.

I imagine that's because I changed the name of the Primary Menu? Is there a php file I need to edit to replace the old name with the new? And where do I look for it?

I don't know if the mobile menu is specific to the theme (we use a child theme based on the Discovery Theme) or if it was something that was coded by the person who built our website.

Also, just on a housekeeping point, we have 4 old menus we don't need but I don't see anywhere to delete unwanted menus. Is there?

I've enabled support access to our site Thanks for your help.