Shocker! Complimentary upgrades are not guaranteed. Why should US upgrade you when they might be able to sell the seat? As noted above, if US does not sell the seats, they will be filled (at some point prior to departure) with complimentary upgrades based on status.

If you want a guaranteed F seat, by an F ticket. I have frequently bought an F ticket at the last minute in situations where I don't have status, or there are only one or two seats left and my odds of getting an upgrade look slim.