Using If's and Elses to fill multiple cells

I am having troubles getting the third and 4th cells in this section to fill properly. I am pulling data from one specific cell on a separate sheet (O1), and attempting to fill one cell at a time, only if (O1) is not the same as a previous listing.


The first and second operation will work, no matter which of the If/ Else bunches that I ask it to fill. However the 3rd and 4th changes will not fill the 3rd and 4th cell. Any ideas?

    If Range("at9") = "" Then
    ActiveSheet.Range("at9") = Worksheets("Mud Reports").Range("o1")
    Else
    If ActiveSheet.Range("bc9") = "" Then
    If Worksheets("Mud Reports").Range("o1") <> ActiveSheet.Range("at9") Then
    If Worksheets("Mud Reports").Range("o1") <> ActiveSheet.Range("at10") Then
    If Worksheets("Mud Reports").Range("o1") <> ActiveSheet.Range("bc10") Then
    ActiveSheet.Range("bC9") = Worksheets("Mud Reports").Range("o1")
    Else
    If ActiveSheet.Range("at10") = "" Then
    If Worksheets("Mud Reports").Range("o1") <> ActiveSheet.Range("at9") Then
    If Worksheets("Mud Reports").Range("o1") <> ActiveSheet.Range("bc9") Then
    If Worksheets("Mud Reports").Range("o1") <> ActiveSheet.Range("bc10") Then
    ActiveSheet.Range("at10") = Worksheets("Mud Reports").Range("o1")
    Else
    If ActiveSheet.Range("bc10") = "" Then
    If Worksheets("Mud Reports").Range("o1") <> ActiveSheet.Range("at9") Then
    If Worksheets("Mud Reports").Range("o1") <> ActiveSheet.Range("bc9") Then
    If Worksheets("Mud Reports").Range("o1") <> ActiveSheet.Range("at10") Then
    ActiveSheet.Range("bc10") = Worksheets("Mud Reports").Range("o1")
    End If
    End If
    End If
    End If
    End If
    End If
    End If
    End If
    End If
    End If
    End If
    End If
    End If