I just heard back from customer service. Apparently I entered the wrong security check point. There are three in SEA. I had gone to the one closet to 'A' gates because that is where Frontier used to be. Now they are in B gates, so I assume I need to go to the check point that is located in the middle of the airport between B and C gates.

Mr. The P,

Excellent question. I called our ticket counter in Seattle and our agents there verified that our Summit and Ascent members still qualify for Priority Security screening, but not all lines are created equal. If you enter security nearest our ticket counter and gate, you'll have the option you want. If you're not sure of the correct location, please ask our agents.