How close are you to departure?

O fares between CDG-SIN have quite some restrictions because they’re highly discounted. For what can I see, many have 60 days of advance purchase requirement, but the less restricted have at least 7 days. It means that if you want to leave in less than 7 days (considering your departure date, even if you want to change only the return, because your change requires re-issue) you cannot get an O fare.

This could be the issue but there could be more.