Could be that the font is in an "empty" text frame, or used in character/paragraph styles. It could be on a hidden layer, or used on the pasteboard, or anything.
But the bigger question, why are you outlining fonts?
Exporting to PDF with live text will embed the font to be used in output.
Outlining fonts is an archaic workflow, there are only certain times it's a good idea, like stylising a font and reshaping it to form a design feature.
There are other legitimate reasons, but outlining generally is not needed.