It is likely that the inbox needs to be pinned to your left-hand sidebar. You can add it with the below steps: 

1. Hover over any line in the left-hand sidebar and click the vertical "..."
2. Click Edit sidebar

3. Scroll down to the Shared inboxes section and click the next to Add shared inboxes
4. Check the necessary boxes in the pop-out inbox list

• This will display the inboxes the teammate has access to

5. Click Save