Hi James,
Thanks for reaching out to us here on our support forum.
Normally this kind of problems are due to your server not been able to connect with Facebook, there are several reasons why that happens. What you can always check is if the event is public and if your API keys have access to that said event.
If now your problem is resolved without much change, I would say it’s the first one and you just need to keep an eye to see if that happens again a good choice may be to change server.
Let me know if I can mark a correct answer and close the thread here.
My Best Regards,