A user was added to a particular group and marked as the supervisor, but the user is not able to see the conversations made by the group members.
A newly added supervisor will only access the conversation made after the exact time of his/her group membership.
In Verba, the group membership of a user can be manually changed and dated back to a specific time. It allows the supervisor to access conversations which were made before the start of the membership.
The group membership can be changed under Group Membership History settings on the group configuration screen. On our Knowledge Base, we have a details article about the Group Membership History feature.