In my experience, flights that don't post are those that were incorrectly noted as "open" (i.e., not flown). In that case, without the boarding pass, if you ask them to look up their records, they'll give you the same reason for not crediting the flight in the first place: "you didn't take the flight."

The boarding pass states something like "retain this document for proof of travel". If you didn't save it, you don't have the proof of travel, and you do not deserve flight credit.

Next time, hold on to your boarding pass!

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"There's no sadder sight in this world than that of a football player trying to think." -- Daria