I'm flying back to PDX today from DEN via SEA. My DEN-SEA flight was on time, but had originally booked a longer connection time in SEA, though wanted to get on an earlier QX shuttle to SEA. All attempts FAILED:

• App (It tried, but got an error message saying it couldn't be processed)
• CSA at the Gate (who did her very best and was very helpful)
• Gold line over the phone (She even checked with a supervisor. They didn't say it wasn't possible, only that it didn't go through.
• AS.com (similar message to App above)

None of the above would allow me to be placed on the Standby list for the flight. My attempts were prior to 11am for a SEA-PDX shuttle at 2:05pm, so what is going on, or what am I doing wrong?

Are SDSB's ONLY done at the gate, in person? Every AS rep I spoke with suggested it was possible, so not sure?

Checked on elite benefits on as.com and unless I didn't look in the right spot, I didn't locate any info that would suggest I'm doing something wrong.