Troubleshooting
Fixes for the things that can go wrong, from a missing screen to an item that won't stay where you put it.
I can't find a screen any more
Open any admin page with ?adminline-safe=1 added to the address: the full menu is back on that page (for administrators). Then go to Settings → Admin Menu, find the item and change its visibility, or use Tools → Reset the menu. See Safe mode and reset.
I'm locked out of the admin
Administrators can't lose Settings → Admin Menu or Plugins. If something else stands in the way, add define( 'ADMINLINE_DISABLE', true ); to wp-config.php, fix the menu, then remove the line.
Someone can still open a hidden screen
Check the item's When hidden setting: Hidden only leaves the screen reachable by its address; Hidden and blocked refuses it. Also check whether the person has another role that may see the item: someone sees an item while any of their roles may.
Blocking covers the screen itself. Saving forms, the REST API and other plugins' screens follow the person's capabilities, which Adminline doesn't change.
A plugin's item is in the wrong place, or marked New
Items a plugin adds after you saved your menu go where the plugin puts them, marked New. Move it and save; it stays put from then on. See New and removed plugin items.
An item I moved went back
The menu you had moved it into belongs to a plugin that's inactive now. The item comes back to where you put it when the plugin is activated again.
A renamed item lost its count
Count bubbles come from the plugin that adds the item, and only show when there's something to count (pending comments, processing orders). If a plugin adds its count in an unusual way, the count may not be recognised; the item still works.
The preview doesn't match what someone sees
The preview is built from your own menu and the role's capabilities. A plugin that adds items only for certain people, or under its own conditions, can make the real menu differ. Log in as that person to check.
The menu editor is empty
The editor needs JavaScript. If it's on and the editor is still empty, another plugin's script error may be stopping it: your browser's console shows the error.