If you don’t want to use a megamenu as suggested by Nial, there are a couple of options to consider… if you’re willing to re-think how you’re presenting your information to users.
You’re using a STICKY header… wherein your entire header (logo and menu) is FIXED to the top as you scroll down the page. This means there will ALWAYS be a FIXED HEIGHT, from the fixed header to the bottom of the screen. And once you have a fixed height, there is a limit to the number of sub-menu items that can be visible above the fold (irrespective of screen size), as you’re experiencing now.
Here are a couple of options to consider:
1) Un-STICKY your header (make it scrollable) so that the additional categories can show on the menu as you scroll down the page. I don’t know if your theme has this option or not though.
2) If still want to keep the sticky header, you can use JavaScropt or CSS wrangling to make the drop-down menu alone scrollable within the drop-down menu’s container, so all the categories can appear upon scrolling. But this is way outside the scope of the WordPress support forum, and you may want to reach out to your theme’s author for help.
TUrned out you can easily do this with CSS:
@media (min-width: 768px) {
ul.dropdown-menu li ul {
max-height: calc(100vh - 150px) !important;
overflow-y: auto !important;
} }
Note: this will only work from your theme’s media breakpoint of 768px AND UPWARDS (ie not on mobile devices). Also, the 150px is the approximate height of your entire header. See screenshot of this scrolling sub-menu below.
3) You can forget about the drop-down menu completely, and, instead, link the CATEGORIES main menu to an index PAGE listing all the categories.
I’m sure there are other options I’ve not thought of. But the real issue here is sticking your menu FIXED to the top means you’ll have a FIXED height to work with, and a finite number of menu items can be stuck in the drop-down menu. So you’ll have make the drop-down scroillable, use a megamenu as Nial suggested, or use no drop-down menu at all.