Maybe this will help:
Selection.Autofill Destination:=Range("F1:F" & Range("A" & Rows.Count).End(xlUp).Row)
Basically what it is doing is checking to see what the last cell with a value in A is and fill F1 to the F whatever the last cell with value in A. The only way this will work is if you have the same amount of data and rows in column A as you need to fill in F.
I hope that makes sense bc I know what im talking about and it doesnt make sense to me. Just try out the code I have to use this for most of my macros.