Reports, letters, mailings may not print on both sides of the paper if your printer is not setup for double-sided printing.
In order to print on each side of the paper, you must have a printer that is capable of double-sided printing. If you have this, when printing, select the double-sided printer and select to print on both sides. If you are unsure where this option is, consult your printer manual or contact the manufacturer.
Print the first side of the mailing and then run the pages through the printer again, using the opposite side to print additional information.