There are a couple of reasons why the PDF is not appearing on your session card:
The PDF was never exported during the lesson (by clicking on the envelope icon in the toolbar) and the material is still in your Room.
The PDF was exported but not during the lesson itself. For example, if you disconnect from your session and then go back into the Room later to export, that PDF will appear on a different session card.
The other user exported the PDF whilst you weren't in the room in a separate session on their own.
If (2) is the case, once you click on the eye in the right hand corner of your Sessions page you should be able to see all of your sessions. Does the PDF appear on any of the session cards adjacent to the taught session?
To prevent this happening in the future, we always recommend exporting work (if you so desire) whilst you are still in that session, whether that be at the beginning, middle or end. This will then always guarantee that the PDF appears on the session card that it correlates to.