That's because your autocorrect options have been set to automatically change
the text combination 1/2 to ½, 1/4 to ¼, and 3/4 to ¾. The autocorrect
options will not automatically change other text combinations (such as 1/8)
to their fraction symbols.
You can EITHER
1. Use the Replace function to change the fraction symbols (like ½) back to
text combinations (like 1/2). If you use this option, turn OFF the
autocorrect fraction option so they don't change back.
OR
2. Use the replace function to change the text combinations (such as 7/8) to
their fraction characters (like â…ž). If you use this option, you can insert
fraction characters from Insert - Symbol, or by using Alt-X combinations that
will be shown in the Insert-Symbol window.
I hope this helps.
Fred