Most likely explanation is that you’ve hit a PHP limit here – namely max_input_vars.
A more thorough explanation can be found here.
The solution is not inside WordPress I’m afraid, you’re gonna have to venture into your site’s PHP settings.
Or, of course, reorganize your menus into something more manageable.
Related Posts:
- Create custom page and add it in nav menu by code
- Creating and populating custom nav menu (menu-item-position not working)
- wp_nav_menu: show menu only if one exists, otherwise show nothing
- Does Extending Multiple Nav_Menu_Walkers Allow Nested Menus?
- wp_nav_menu remove class and id from li
- Add separator to admin submenu
- On which hook should I be calling register_nav_menu(s)?
- Add child pages automatically to nav menu
- Automatically remove trashed pages from nav menu
- Directing a page to a default subpage
- Menu is not visible in appearance
- One site, 2 distinct sections: two different top nav menus?
- Can you have different menus and theme on a per-page basis?
- List children of second level sub page
- Make Theme automatically choose default nav menu
- How to make pages metabox display all hierarchy in menu admin?
- How can I disable parent menu item links?
- How to get current page menu item name instead of full menu item list
- Organizing by super-categories (or nested categories)?
- Nav menu items disappearing (but not immediately)
- Hide pages depending on role
- Get parent page/menu id of current post
- wp_nav_menu custom walker – child items
- Change menu items URL
- wp_nav_menu doesn’t generate parent/ancestor classes
- Making WordPress’ page and menus visible only for logged in users, checking session variables
- Add static item to wp_nav_menu
- Different side menu on each page
- How to get wordpress nav menu outside of wordpress system?
- WordPress – site with 2 stylesheets and more…
- Add nav menu CSS class to body
- Remove Page Name From WordPress Menu
- Hiding Menu From Specific Pages
- Change HTML structure for custom menu in sidebar
- Getting post content within wp_nav_walker
- wp_list_pages sort order is different for different languages [closed]
- Using Shortcodes in WP-Menus in WP 3.1 (via nav_menu_objects)?
- list pages only from master parent
- Custom Admin Menu Order
- Current menu item
- wp_list_pages: only show subpages on the parent page?
- remove everything from wp_nav_menu
- How to add wechat (weixin) link to navigation menu?
- Auto add pages to primary menu with functions.php
- Admin only menu/page
- Does loading of sub pages in menu cause load to the server?
- Best way to customise wp_nav_menu [duplicate]
- WordPress menu permalink redirects to wrong page
- Link to Homepage in Menu
- Schedule Page to Menu [duplicate]
- How to add custom classes to more than one separate menu’s a link in wordpress?
- Add class active to static menu
- How do I separate link levels in a custom menu walker?
- SlideDeck 2, make back-end visible for admins only
- wp_list_pages() but only show children on the branch you are on
- Check if Page Already Exists in Menu
- Programmatically add existing pages and subpages to Appearance Menu
- How to retrieve parent menu item name of current submenu item
- two different separate sections on the same site
- When clicking on menu item not going to specific page
- Ad custom admin page like WordPress customizer [closed]
- Displaying different page links on different pages
- pull non-wp site into content area of wordpress -or- pull wp header into non wp site?
- Help with walker for nav
- Using my WordPress menu on another platform
- Remove a menu sub-item
- Does is_child() exist in wp 3.5.1?
- How to add html attributes to the “ul” html tag with “wp_nav_menu” function?
- Using _s theme, menu changes do not affect header menu
- wp_list_pages sort order by custom nav menu order
- Limit User Iinterface for Admin?
- How to highlight 1st level menu item based on actual page
- How can I switch pages?
- Make pages visible to only logged in members
- Prevent menu from loading in a page template [duplicate]
- disable Tab post on nav-menus page (Admin)
- Get custom classes in WordPress Navigation Menu
- How to create a menu item which is not a page
- How can I make a menu link visible only to admins (without plugins)?
- How to add a page to header that is outside of WordPress?
- Replace the slug of parent pages with # [closed]
- How to Structure Pages and Subpages
- Highlighting top level link when viewing one of it’s sub-pages, which are not linked in the menu
- Three menus – parent / child / sibling
- Insert menu with a custom walker into page / post body using shortcode?
- Add (grand)child pages automatically to nav menu
- how avoid the link of the current post in the menu in the sidebar?
- Disable parent in wp_list_pages – WordPress
- Page appears after de registering from menu
- Get parents child pages
- How to undo the last changes to a custom menu?
- Dynamic menu wp_list_pages displaying only current branch including: current page, his siblings and its childs
- wp_nav_menu without menu or theme_location specified
- Filter to hook into menu items and add a custom attribute
- wp_list_pages Hierarchical Help
- Responsive menu on genesis theme [closed]
- Parent Page > Child Page Menu
- Making pages in a menu appear conditionally
- Add pages content to startpage through custom menu
- add_menu_page does not render the expected result