what is the definition of the index?
your query has three columns in the WHERE statement, is the index on all three columns?
since you are doing SELECT *, at a minimum, there's going to be a key lookup to get the other columns, so the optimizer might decide the cost is less to use the clustered index; more details on the existing indexes is needed i think.
WHERE (SourceID = 'CCH')
AND ([RowUpdateDateTime] >= @RefreshDate)
AND (NOT [ResultDateTime] IS NULL)
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