A Facebook event was created and is being co-hosted by two organizations that have two different nations. The Facebook event is connected to one event page per nation, RSVPs from Facebook are recorded in on one nation's event page and not the other.
Connecting a Facebook event to your nation requires that you authorize through Facebook. This process happens automatically when you select the button that says "Click here to allow us to manage your Facebook events". Be sure that you are logged in to the correct Facebook account when you go through this process. I recommend disconnecting and reconnecting the event and try to re-authorize. For more information see our documentation on connecting a Facebook event.