In Notepad++, installing plugins via Plugins Admin…(plugin manager) may show like the plugin was installed, but when navigating to Plugins menu items the plugin will not be found there. This could be mainly due to two reasons. One reason is that Notepad++ is not opened in Administrator mode. The second reason could be you are behind the company's proxy.
Before doing the below settings, update Notepad++ to the latest version.
Start Notepad++ in Administrator mode to configure the proxy.
If you know your company’s corporate proxy IP and port, the proxy has to be set at the question/interrogation mark symbol menu,
? -> Set Updater Proxy…
If you installed PX Proxy, the IP address will be localhost and the port will be 3128.
On setting the proxy via Set Updater Proxy, you are all set to install any plugins via the Plugins Admin menu item.
Behind a company proxy, if you haven't set the proxy this will cause the problem.
Sometimes if the proxy is set in Microsoft Edge internet browser, notepad++ may take proxy from Edge Proxy settings also.
After Setting the proxy, Notepad++ will restart and plugins will now appear in Plugins Menu.
This solution is tested in Notepad++ v8.4.4.