How can I switch tabs in a WinForms TabControl in designer?

I have inherited code that includes a Windows Form that contains a TabControl with 5 tabs. I can't switch tabs in the designer. When I open the form in design view, the first tab is selected, and the contents of the first TabPage is displayed. When I try to switch tabs, the correct tabs at the top are selected, but only the first TabPage is displayed. I can add tabs to the control, but they don't display with an empty TabPage to work with. How can I switch tabs in the designer view so I can modify the content of the second or third TabPage?