I've had this issue with some of my Mac apps. You try to touch on a point with your pen and it gets wonky and jittery. It doesn't stay in place for a long time. It doesn't register clicks and if so, only for a slight moment. Right click is a joke. I've had this issue with apps like Word and Rdio as well. Photoshop, Illustrator, InDesign, and Chrome have been fine.
I was wondering whether it was a sensitivity issue on the tablet? Are the Sketch designers only using mouses to design? I prefer the speed of a tablet. It feels more engaged and intuitive than a mouse being dragged across a surface.
Has anyone else faced this issue?