VBA Vlookup Problem

I use the following code for a vlookup in my VBA Macro:

    Range("F2").Select
    ActiveCell.FormulaR1C1 = _
        "=IF(ISNA(VLOOKUP(RC[1], Lookup!A:A,1,FALSE)), ""Not Found"", ""Found"")"
    Selection.AutoFill Destination:=Range("F2:F65000")

When this is executed in Excel, the following results show (attached picture) incorrectly. The vlookup isn't calculating?!

But then outside of the macro, when I click in the formula bar and press 'enter', it calculates it to a different result. Why isn't it calculating correctly with this code?

I'd appreciate any help, guidance, or direction someone can offer.

Thanks