Found the problem !
Conflict with the W3 Total Cache plugin.
Thank you for the quick response, Tikaram ! Greatly appreciate it.
Still having the same issue – I think.
I went through and deactivated each plugin, one by one, and cleared the cache after each deactivation. Same problem of menu not expanding on mobile so I am not able to pinpoint a problem plugin. I did, however, delete some plugins that I no longer use and cleared the cache again.
I did notice that if I reduce my desktop to resemble a mobile-size screen, refresh the page and then click the menu ‘hamburger’, it seems to work.
So…perhaps it’s a timing problem ???
I’m still not sure why this would happen.