I assume that you read Creating fractions on the MVP site. If you have not, you need to do so.

You need to set it up so that all of your fractions are produced in the same way. That means first to turn off the AutoFormat as you Type option to convert fractions.

Apparently you have not created the AutoCorrect entry or you saved it as unformatted. What fraction symbol, in particular, is causing the problem? Is it a font symbol? An equation? A combination of superscript/subscript?