First, if you are using a Windows driver for your printer make sure you have the correct driver selected in the printers list. If you plug a Windows printer into a different USB port than it was originally installed on Windows will add a new copy of the driver and call it Copy 1, etc.. You can see this in the Devices and Printers section in the Windows Control panel. If you have Booth set to the original driver then from Booth's perspective the printer is not present. It is best to always plug a printer into the same USB port every time to prevent this. To find out which driver is the correct one try printing a Windows test page and see if it prints.

Next, in the Global settings section in the Printers list double click on the printer and make sure the Page Size (lower left corner of the properties window if you are using a Windows printer) is set to 4x6 if you are printing 2x6 or 4x6 prints. For any other size set it to that size (for example, 5x7 print set that to 5x7 page size). For 2x6 size make sure the "Allow Multiple-Page Documents" is check as well.