Please note that this article refers to RKSV receipt signing and is therefore only relevant for stores in Austria. If you operate a store in Germany, you don't need to perform a subsequent recording if fiskaly fails.
If receipt signing fails due to an issue on the part of fiskaly or FinanzOnline, you'll notice this both from a notice on the receipts themselves and in the top left of the app.
In this case, the receipts need to be recorded retroactively. To do this, click Jetzt nacherfassen (Record now) in the notification and then click Nacherfassung starten (Start recording). The retroactive recording now runs automatically. Once it's complete, click Fertig (Done). No further steps are necessary.