I suspect some plugin is hooking into the
user_has_cap filter and removing the
edit_plugin cap for the administrator account.
To find out if this is the case, try deactivating plugins, one at a time, and see if the editor comes back. When/if it does, you'll know which plugin has removed the cap.
What you do once you find the plugin in question is a different matter. If the plugin is vital to the functioning of your site, then see if it has a setting where you can tell it not to remove the
If it doesn't, then do NOT edit the source of the plugin to fix the problem (because your edit will get lost when/if that plugin's author updates it). Instead, you can simply hook into
user_has_cap yourself (in a plugin or theme you control) and add the cap back in. If you do this, make sure you are hooking in with a higher priority than the plugin in question is using, e.g.,
add_filter ('user_has_cap', 'your_func', 99999, 4) ;. See user_has_cap and add_filter() for more info on how to do this.