If you have verified that the subscription endpoints are normal, the most possible reason is that the subscribers have not confirmed their subscriptions or the message has been blocked.

Check whether the message is blocked and processed as a junk message.

If the problem persists, contact technical support.

SMS Message

If the subscribers fail to receive the confirmation message, check whether the message has been blocked.
If the subscribers fail to receive the notification message, check whether they have confirmed the subscription or whether the message has been blocked.

If the problem persists, contact technical support.

Why Do Subscribers Fail to Receive Messages After I Publish Messages to a Topic?

SMS Message