Ligatures are combinations of letters that are treated as a unit.
Some versions of fonts and some printer drivers do not have the ligature characters available after a default installation. If the characters are appearing correctly in the preview but fail to appear when printing, then you are experiencing a problem with your Windows printer driver. After selecting Preview from the Typeset menu, select Print Setup from the File menu and then choose the Properties button for the printer that is being used. The dialog that comes up at this point is different for different printer drivers, so specific help can't be given. However, you should look for settings related to the printing of fonts.