i know the quill/adapter isn't slipping because it has an inset bolt which pretty much leaves a pool to collect water and seize the bolt, so i glued a george washington dollar over it to keep the water out and his head doesn't spin when the bars do.
i've tightened the stem to neck of the adapter damn near to where i'm stripping the allen heads so i'm sure it's tight. i'm just gonna go with the explanation that it's too slick surface to surface.

i'm pretty sure it's the delta adapter, i got it for $10 when i was in a pinch out of state.