[PeterT]

Nope... Many of the Kobo fonts do not contain all characters.

Try using Ryumin or Gothic... I've had some success with those.

I've been raising the point that not even fractions seem to be recognized in all the fonts.

Amasis: 1/4, 1/2, 3/4, rest are a rectangle with a x inside
Avenir Next: 1/4, 1/2, 3/4, rest are a rectangle with a question mark inside
Caecilia: 1/4, 1/2, 3/4, rest are a rectangle with a question mark inside
Georgia: 1/8, 1/4, 3/8, 1/2, 5/8, 3/4, 7/8, rest are a square
Gil Sans: 1/4, 1/2, 3/4, rest are a square
Kobo Nickel: 1/4, 1/2, 3/4, rest are a square
Malabar: 1/4, 1/2, 3/4, rest are a rectangle with a x inside
Gothic: 1/8, 1/4, 1/3, 3/8, 1/2, 5/8, 2/3, 3/4, 7/8 - Correct
Ryumin: 1/8, 1/4, 1/3, 3/8, 1/2, 5/8, 2/3, 3/4, 7/8 - Correct
OpenDyslexic: 1/4, 1/2, 3/4, rest are an empty rectangle with a thick bottom line

You could also try using the Kobo Extended driver and converting this to a kEpub. The kEpub renderer handles missing symbols differently, and displays them from one of the other fonts (Ryumin I think).