max_input_vars
The most common issue stems from the max_input_vars PHP directive, which was introduced in PHP 5.3.9. By default, max_input_vars is set to 1000, which can easily be reached with the WordPress menu system, since each menu item sends about 12 variables per item.
Solution
The solution is to simply configure the max_input_vars so that it is large enough to handle the number of variables being sent by WordPress to save your menu.
php.ini
If you have access to your php.ini, you can just add this directive (or change the value of the existing directive) to includes your max input variables:
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max_input_vars = 3000;
If you don’t have access to this file (as with many shared hosts), you may need to contact your host to increase this limit.
.htaccess
Alternatively, you can try placing this in your .htaccess. This won’t work on some servers, so your mileage may vary.
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php_value max_input_vars 3000
.user.ini
If you have a .user.ini file, you can add the following to override the default php.ini value
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max_input_vars = 5000;
原文连结：http://sevenspark.com/docs/ubermenu-3/faqs/menu-item-limit