When You upload new report in ssrs server,DataSource mapping to report is not defined.
You can check by
-- Shared Data Source references with broken links
SELECT
ItemPath = c.[Path],
ItemName = c.Name,
DatasourceRefrenceName=ds.Name
FROM
[Catalog] AS c
INNER JOIN DataSource AS ds ON ds.ItemID = c.ItemID
WHERE
ds.Link IS NULL
AND ds.Extension IS NULL