Why is it so hard to upgrade to or from SFO? I just completed my third trip to SFO last week on a H fare with a K return and failed to upgrade. When I left PBI on Monday I was number 3 with 8 seats open on the flight into SFO when I got to IAH I was something like number 9 with all F seats filled. I asked about my chances at PBI and they told me it looked good to upgrade so I declined to change my Y middle seat thinking I would be upgraded (big mistake kids). The IAH PC agent told me that there were many full Y's sold so that is what bumped me down the UG list. Can anyone shed some light on why it so hard to score an upgrade even on H fares? Thanks.