I found similar with a chrome set of handlebars, there's lack of grip to a shiny chrome surface so I smeared a tiny bit of valve grinding paste in the 2 half moons of the stem and gently clamped the stem in the correct position and moved the handlebars back and forth, tightened it a little more then done the same again.
I then removed the bars ,cleaned the stem / bars thoroughly , it made the bars hazy in that area but now they grip.