Originally Posted by sbm12
Buying a Y doesn't guarantee you the U/G, even if there are seats up front. If the flight has the golden U icon online then you'll get the upgrade. Otherwise it is a toss-up. I have bought many Ys (admittedly a high-rev market) and not gotten the U/G, as I am only a lowly gold elite.
Most likely this is because F is sold out. Elite Y upgrades come from A inventory, so if there is A available, it's yours. Always check for flights with availability in the F cabin before purchasing full-Y to be sure you get up front.
Robin