Seems like your retention rate is 7 days, Check with retention periods for Session and Account info collectors.
Admin->Teradata Systems->Select monitored systems->Collectors->Sessions and Account Info collector->Check Delete Data policy.
For Viewpoint 15 and later
Settings->Monitored systems->Select monitored systems->Collectors->Sessions and Account Info collector->Check Delete Data policy.
If its is more than 7 days, I would recommend you to open a Incident with Teradata
Another thought, some folks are under the impression that history data is bulk loaded from Teradata when you first start using it, which is not the case. History starts at the time the first collection is initiated. So for instance, a brand new Viewpoint implementation will have zero history data.