Please can you tell us a little more about your environment?
Are those servers somehow connected? failover cluster, AvailibilityGroups?
What is the error message that you get?
What does the error log of those SQL instances tell you?
What has changed since you initially set up the Azure backup for those instances?
Did you try to re-discover those instances and initialize those backups again (from the Azure backup side)?
What steps have you taken to check those backups and rerun them?