IF statement in VBA not Working & not sure how to fill down an IF Then statement

[I am a novice with VBA all self taught. I have been trying to get this If Else Then statement to work and no matter what I do it doesn't do anything. It's not giving me any errors so the macro in VBA is running but it doesn't complete the task I am asking it to. I also need the macro to fill the If then Statement down to the bottom of the sheet and I'm not sure how to go about doing that. Any help is appreciated.]

If Range("A17").Value = D01 Then
    Range("S17").Value = "I"
ElseIf Range("A17").Value = S01 Then
    Range("S17").Value = "I"
ElseIf Range("A17").Value = D02 Then
    Range("S17").Value = "O"
ElseIf Range("A17").Value = S02 Then
    Range("S17").Value = "O"
End If