There is a bud in the GUI of Office 365 when delegating full permission to a user for shared mailbox.
Even if you give the correct permission you cannot expand the mailbox.
The fix is to remove and readd the permission with these powershell comamnds:
To Add
Add-MailboxPermission -Identity JDoe -User ‘testuser22’ -AccessRights FullAccess -InheritanceType All -AutoMapping $false
To Remove
Remove-MailboxPermission -Identity JDoe -User ‘testuser22’ -AccessRights FullAccess -InheritanceType All
Reklamer