A common issue is a user who has a role higher than agent only being able to see the same things as an agent.
You might have made someone an admin in Settings>Users and Workspaces>Account Users but they report to you that they can only see their own tickets and data.
This is usually because they have been restricted to the agent role in a workspace. This allows you to have a user as an agent in one Workspace but a higher role in another. However, this sometimes can cause confusion.
If you have a user who should be able to see more than they can, you can follow these steps:
Go to Settings>Users and Workspaces>Choose Workspace
Check if you can see this next to the user name:
If you don't want them to be an agent here, hover here and remove restriction: