UDF Not Updating As Expected

I have a written the function below, but when ever I use it, and for example drag it across lots of cells, they all come up with the same value, and I have to manual click on each one and pres enter to get it to show the right value. I have tried searching but without much luck as I am not sure what I should be searching for. Using application.volatile doesn't.

Option Explicit
Public Function FirstLinePickUp(inputrow As Variant) As Variant
Dim n As Integer
Dim testcell As Variant
n = 0
testcell = ""
Do Until testcell <> "" Or ActiveCell.Column - n <= 0
testcell = Cells(inputrow.row, ActiveCell.Column - n)
n = n + 1
Loop

FirstLinePickUp = testcell

n = 0
testcell = ""
End Function