USA has fulltext access to the journals in the IEEE All-Society Periodicals Package (ASPP), 1997-current and a few other items only. To be sure to find materials with available fulltext, limit your search to "My Subscribed Content" as shown on this page.
You may still search for conference proceedings, IET journals, and other content in Xplore, but full-text is only available for the ASPP journals. On results lists, look for the green unlock icon for available full-text.
From the IEEE Xplore homepage, choose "Advanced Search," then choose the "My Subscribed Content" radio button:
On a result list, look for the green "unlock" icon to tell which items are available in fulltext: