iPhone 5 screen won't rotate in Numbers app

I have the latest version of iOS numbers (version 1.7.1) installed on my iPhone 5 and iPad 3. The screen automatically rotates on my iPad, but not on my iPhone. This is frustrating and makes viewing spreadsheets much more cumbersome on the iPhone. Does anyone know if there is any way to allow Numbers to autorotate the screen on an iPhone?

Thanks

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