plugin manually installed doesn't show up

[gghh]

Hi. I just installed a plugin manually (uploaded plugin folder to wp-content/plugins), but the plugin doesn't show up in the admin dashboard.

I tried some basic troubleshooting:

• the .php file of the plugin has a well formatted header
• set the file and directory permission correctly (744, owner of the files is the apache user www-data)
• compatible version of WP
• checked the support forum for similar problems, etc

I am surprised because I have 5 other plugins, all manually installed, all working.
I have placed the plugin folder, named table-of-contents-plus/ , under wp-content/plugins

The plugin version I have is 1509 (downlodaded from https://wordpress.org/plugins/table-of-contents-plus/ ), WP version 3.8.2