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Check all the settings on your printer for Paper Size, and/or Print Size and/or Paper Source and/or Paper Stock. If any setting is found to be of lesser size than 16x20, correct it.
On your computer, go to the driver settings. (To be told exactly where the driver settings are, tell me whether your computer is a Windows PC, or an Apple computer, please.)
In the driver settings, make sure the Paper Size, Paper Stock, Paper Source are all correctly sized for 16x20 printing.
Lastly, in your program that you are printing from, when about to send the print job, make sure Paper Size, Paper Stock, and Paper Source aren't any of them wrong.