Populating previous value

Hi I have a SQL QUERY

SELECT ID,ApptDate,OpDate,SacSize,
    ( SELECT TOP 1 b.SacSize
      FROM Table.vAllData as b
      WHERE b.ApptDate< a.ApptDate
      ORDER BY to_number(b.ID) Desc
    ) AS Prev_Value
FROM Table.vAllData a where HospNumber='N900001'

the above query Outputs the below, but unfortunatuly i cant get the 'previous_value' column working to populate from the second row, am i missing some thing?

many thanks