Code works in macro but not command button

Hi there
I've got the code below which works fine when I run it from a macro but when I put it behind a command button it only puts an 'A' into row N2 and no further. I wonder if anyone can explain why.

Thanks, as always, for any help anyone can provide.

datafiend

Worksheets("Class A").Select
    Dim lastRow As Long
    lastRow = Range("C" & Rows.Count).End(xlUp).Row

    ActiveSheet.Range("N1").Select
    ActiveCell.FormulaR1C1 = "Classifier"
    ActiveSheet.Range("N2").Select
    ActiveCell.FormulaR1C1 = "A"
    Range("N2").AutoFill Destination:=Range("N2:N" & lastRow)