Not all fractions are available in every font.
Some only have half, quarter and maybe three eighths. Others have one third
and two thirds.
Check in insert > Symbol. Go to (normal text) on the left of the dialog and
Number Forms on the right. This will show you which fractions (if any) are
available in your chosen font. As a general rule, you get more fractions in
Times New Roman or Arial than you would in a custom font like Garamond MT.
You could try using equation editor to create fractions, or you can "fudge"
them using superscript and subscript, but they don't look as good as the ones
that come with the fonts.
Genine