In your Case statements, you should be comparing to the value in your "Select Case" field which is SiteSurveyStatus. Not sure you need to use Case, maybe try:
If Not IsNull(Me.SurveyComAd) thenMe.SiteSurveyStatus = "Completed"
ElseIf DateDiff(("d", Me.SurveyStAd, Date) And (Me.SurveyStAd) > (Me.PlanSurveyStartDate)) Then
Me.SiteSurveyStatus = "Delayed Progress"
Else
If DateDiff("d", Me.PODate, Date) <= 0 And IsNull(Me.SurveyComAd) Then
Me.SiteSurveyStatus = "In Progress"
Else
If DateDiff("d", Me.PODate, Date) > 0 And IsNull(SurveyComAd) Then
Me.SiteSurveyStatus = "Delayed"
Else
Me.SiteSurveyStatus = "Not Scheduled"
End If
End If
End If
End If
Put a break point on the first If line and step through the code to see if it is doing what you want it to do. You can hover the mouse over the fields to see what values they hold.