Dashboard not reading all installed plugins

My newly installed version of WP4.0.1 is not reading all the plugins installed in its /wp-content/plugins directory. I’ve tried removing and re-uploading the plugins, but that doesn’t seem to work. The only plugins the dashboard does read are those plugins uploaded from the dashboard’s “Plugins-Add New” page. When that’s done, a message appears stating the plugin already is installed, but installation of that plugin still does not appear on the “Installed Plugins” page. Any recommendations?