If you have an issue adding additional menu items, then it is likely you have reached the limit for menu items. Thankfully this is an easy fix.
To fix the issue, you need to edit your PHP.ini file. If you are on a dedicated server or on a VPS with root access, then you should already know what we are talking about. If you are on a shared hosting or don’t know where PHP.ini file is, then ask your web hosting provider to do this for you.
You need to ask them to set the following values in the PHP.ini configuration:
suhosin.post.max_vars = 5000
suhosin.request.max_vars = 5000