First check the device configuration to make sure CU*BASE knows the printer is capable of duplexing.  Using Tool #326 CU Hardware Configuration, choose P for Printer and double-click on the printer ID.  The Duplex setting should be checked.  

If this setting is already correct, you will need to contact a Systems client service representative and we will verify that the Duplexing option is set to Yes in the credit union configuration library for that printer model.  (Keep in mind that this may not be possible if the printer model is unsupported by CU*BASE.)
 
If documents from other Windows applications were also not duplexing, you would also need to check the print driver on the PC.  Also, if the printer has a display screen physically on it, there are usually menu options on the printer itself to select/deselect duplexing.