Microsoft Graph WebHook not getting Notifcation

We have active subscriptions created with a GraphWebHook and everything has been running fine for last 4 months. Suddenly today we do not receive any notification from Microsoft o365 server. What could be the problem and how can we trace the issue. Fiddler shows nothing is sent to out WebHook when we change in outlook.

Any help greatly appreciated.

Thanks

Anamika