Installing Jenkins plugin manually

We used Jenkins as our main CI management tools. It worked fine. But today I found two Jenkins can't upgrade/install plugins.
No way to fix this problem, because we didn't change the configuration , they worked fine before.
After searching in google for a while, I found I can install plugins manually. Let's do it.
Refer to the wiki:https://wiki.jenkins-ci.org/display/JENKINS/Plugins
Log into Jenkins server via SSH, we have tomcat7 installed on Ubuntu server. The Jenkins home folder is located at /usr/share/tomcat7/.jenkins
Enter the plugins folder under Jenkins' home folder.
Download the plugin's hpi file here, e.g.
wget http://mirror.xmission.com/jenkins/plugins/pathignore/0.6/pathignore.hpi

Then reload Jenkins app in tomcat7 admin page, go to the Jenkins page to watch if the plugin is installed or not. I can't see the pathignore plugin in installed tab. Why?
Because pathignore plugin depends on another plugin called ruby-runtime.
Download this plugin using the same way.
wget http://updates.jenkins-ci.org/download/plugins/ruby-runtime/0.10/ruby-runtime.hpi

Reload Jenkins again, it works.