What you have
You have a drop-down menu with tons of links. The issue is that Brooklyn applies a max-height of 700px to the expanded drop-down menu in the drawer. That may cut out a lot of links. That max-height is necessary for the drawer animation to be smooth.
What you need
Change the max-height to a value that's high enough, i.e. 2000px, or 3000px.
You may also want to apply a multi-column layout to your drop-down menu in the desktop view, which is another subject...
How to get it
Open the theme's theme.scss.liquid stylesheet in the online editor, use Cmd-↓ to scroll down to the end of the file, and add this:
.mobile-nav--expanded+.mobile-nav__sublist {
max-height: 2000px /* was 700px */;
}