You have a custom CSS Rule
.menu-primary .wvrx-menu, .menu-primary .wvrx-menu-container {
height: 70px;
padding-top: 10px;
}
That fixes the height opf the menu, preventing it from expanding on mobile. I am not sure where you have that rule, it could be either in the theme Custom CSS Rule box, or in one of the CSS+ boxes of the menu options
I understand you are doing that to increase the height of the menu bar, so you have several options depending on what is desired on mobile.
I suspect you can just replace the height by a padding bottom to do the same thing without the side effect
.menu-primary .wvrx-menu, .menu-primary .wvrx-menu-container {
padding-bottom:20px;
padding-top: 10px;
}