When working on card design that has Front Inside and Back worksheets to add images and text and send for printing all work placed on Front and Back worksheets will be printed on one side on the paper (in Printer menu this is called front side) and all work on the Inside worksheet will be printed on the other side of the paper (in Printer menu this is called back side).
In order to print the inside of a project, please select Only back side in section Print Range of printer's options.