I created a simple portlet. I added a Tab to the Overview Page. When I try to add the Portlet to the Tab Content, the portlet was not in the list.
The portlet was created with a Portlet Instance Type = Project. This means that the Portlet is associated with the "Project" Object. The Overview Page is a Portlet Page with a Type = 'Page with Tabs'. You can only see the portlet that was created with a Portlet Instance Type = Project on a Portlet Page with a Page Type = Project. There are two standard Portlet Pages with a Page Type = Project; the "Project Default Layout" Page and the "Program Layout" Page.
Add the Portlet to the "Project Default Layout" Page, Properties, Tabs, Dashboard, Content. To add portlets to the Overview page layout, create a portlet with a 'General' Portlet Instance Type.
This behavior is the same for any Portlet where the Portlet Instance Type is associated with a particular Object.
If a portlet is Restricted you may not be able to add the portlet to other pages within the application regardless of the Portlet Instance type.