System is running into some kinks...
I was denied access at the United Club East in Denver while using a S+ card. I gave them a Delta boarding pass and the agent said they couldn't put my Delta BP into the system. He called the supervisor and she said I wasn't eligible for access and he said it was because "we're implementing a new process".
Anyway, some agents and supervisors simply are unaware of Amtrak S+ benefits and it can be annoying.