Plugins not shown in Updated or Available sections of Jenkins CI tool

I have seen cases where the plugins are not displayed under the tabs Update and Available. It can be achieved in any of the following ways.

Prerequisite:
Make sure that you add the JDK, Ant and Maven installed/referred under the Manage Jenkins ---> Configure System.

Fix:
1) Go to: Manage Jenkins ---> Manage Plugins ---> Advanced, then click Check Now in the bottom right-hand corner.
2) See if the textbox is filled with the entry http://updates.jenkins-ci.org/update-center.json in Manage Jenkins ---> Manage Plugins ---> Advanced ---> Update Site