What is the query, and how are you making the fraction?
Sorry, I should have given this info in the request:
In Character styles, I've checked Fractions in the OpenTypeFeatures
In Paragraph Styles Grep code: \d/\d
I type 3/8 and nothing happens. If I change the 8 to a 4 it accepts the grep fraction style Same thing with 1/4 vs 1/8. Grep doesn't like the 8
What is the font?
Does it convert to a fraction properly if you type 3/8 and manually apply the OpenType fractions attribute?
Unfortunately, the rules for what the OpenType Fractions function ought to do are not set in stone. Some fonts only translate the literal strings "1/2" "1/4" and "3/4" to the correct glyphs ¼ ½ ¾ (and only if these characters are available in that font), while other font designers go nuts and include code and glyphs in the font to allow any fraction (Minion Pro is such a font). It seems you have a font that does the first.
Perhaps this could help you: http://indesignsecrets.com/the-best-way-to-make-fractionsever.php
Thanks for the suggestion. Using calibri in the character style was a simpler, undetectable, and free solution.