Maybe your polygon is too small. When the nodes are too close you will have a very hard time selecting in between them to create another. Look for a node to appear then you know you got it.
Click the little arrow to minimize the component and hide the outline. I keep my collider and outline on separate gameobjects to avoid conflicts.
Thanks for the reply.
I don’t believe the polygon size is the issue.
If I move the polygon vertices way far apart, shift-clicking on the line segments still does nothing.
It’s not really shift-clicking, it’s holding shift and then clicking where you want a new vertex, and then dragging (just clicking won’t do it). If you collapse the sprite renderer component, the outline is hidden. You can also switch to the rotate or scale tools.
All you need to do is zoom in a bit and you should be able to make new vertices. And the sprite outliner can be hided by changing your scene control tools in the top left of the UI from the arrows(blue) to the hand(red)