"Install" option is not appearing in ubuntu software center in ubuntu 13.04

"Install" option is not appearing in ubuntu software center in ubuntu 13.04

Recently I installed ubuntu 13.04 (x64 bit) and i am new to linux. In
"ubuntu software center" the right side "install" button is disappearing
when i am trying to install software. Instead of "Install" button "Use
this source" button is appearing.
"Install" button was appearing previously, but not now.
already tried with below two commands, still "install" button is not
appearing.
sudo rm /var/lib/apt/lists/lock
sudo rm /var/cache/apt/archives/lock
Please help me how can i get back.