Sub dynamic_autofill()

LastARow = CStr(Range("A" & CStr(Application.Rows.Count)).End(xlUp).Row) '<--gets the last used row in the A column

                        '<--are you missing selecting the range("Z10").select here?
ActiveCell.FormulaR1C1 = "=CONCATENATE(R1C2,RC[-25])"
Range("Z10").AutoFill Destination:=Range("Z10:Z" & LastARow) '<--autofills until the last used row in the A column
Columns("Z:Z").EntireColumn.Hidden = True

Range("W10").Select
ActiveCell.FormulaR1C1 = _
"=VLOOKUP(RC[3],'[ASW Rate Plan Validation.xlsx]ASW Rates'!C1:C8,7,FALSE)"
Selection.AutoFill Destination:=Range("W10:W" & LastARow)

Range("X10").Select
ActiveCell.FormulaR1C1 = "=RC[-3]-RC[-1]"
Range("X10").AutoFill Destination:=Range("X10:X" & LastARow)
Range("X10").Select

End Sub