Hi Pat

It sounds like you’re setting the menu correctly via the appearance>menus tab making sure the mobile option is checked at the bottom of the menu page.

I know wpengine do have some sort of caching setup, something like a staging area and a live area. So I would double check that the changes are being published and that they’re reflecting on the live site as well as the staging area.

I would advise testing for a plugin/code conflict by disabling any third party plugins to see if this has any affect.