Tough situation if no warrantee help.
What shop owner/mechanic and I hypothesized was the internal seat tube was wider at the lower points,but narrower up near top. So if it slipped just a drop, you lost your torque, and it slipped further. For us, with trek guiding us, a replacement post and carbon paste didn't help.
They didn't suggest wrapping anything around the wedge, but that is what I would have tried, maybe some kind of rubber or shrink wrap.
You didn't get this from a trek dealer? Even if it was leftovers, original owners should get frame warrantee.
I wish you luck.