A help desk is staffed by members, managed under its Members area.
Tenant members must be added to a help desk individually. Having access to the Help Desk module is not enough to see a specific help desk's tickets — an agent only sees a help desk's tickets once they're added as a member of it. This is deliberate, so that (for example) a brand's support agents don't automatically see another brand's tickets.
Assign each member the role appropriate to their responsibilities, so they can see and act on the right tickets. Add your agents here before expecting them to pick up work in this help desk.