Owner permissions for security group not allowing access

Occasional Visitor

We have a resource mailbox calendar that has a security group ("group A") set as an owner of the calendar folder. Members of that group are still unable to see/add calendar items as an owner. Another group ("group B") is also added as owner and I'm a member of that group - I am able to see/add items as an owner as expected. Any thoughts on why the trouble with the "group A"? Both groups are mail-enabled Universal Security groups in AD. 

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