No option to add Low level items under a top level item in Installed base

Hi Friends,

I am trying to create a new installed base. Everything is working fine but while adding an item and adding some more items under an existing top level item I can't add them. The option to add a lower level item is grayed out. Plz. help me how should I fix this?

   No option to add lower level item

Regards,

Saurabh