Let’s say you have created a nice menu for your theme, saved it and then viewed it on the front end, only to see that a portion of your menu items are not showing. Or maybe even the entire menu does not show. You have assigned it to the main menu, and saved it, but then it drops out and will not show on the front end.
What happened? Did the menu items disappear? Is this a theme issue? Or a plugin issue?
The answer is no, it’s a server configuration issue. Please read this detailed article written by CHRIS MAVRICOS (better known as SevenSparks) to help understand why this can happen, and what you can do to fix it: http://sevenspark.com/wordpress/menu-item-limit