System field 'Created By' does not populate value

Hi All,

In a customized form, system fields "Created By" ,"Created Date", "Modified By", "Modified Date" fields are displayed. When a new record is created sometimes these fields does not have any value. What could be the issue ?

Also, the data source's method 'ModifiedField' is overridden at table level for  a date field. When value is entered for this date field, the system fields does not get populated, otherwise the system fields getting populated. 

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Regards,

  Raghav.