Outside of what is stated in the manual, can't help without seeing an example of what you've done. It works fine for me.
[quote:18g19nlv]Penumbra rendering only works in WebGL mode - in canvas2d mode it will revert to a radius of 0 with hard-edged shadows.
The shadow casters can only use convex collision polygons. Shadows will not render correctly if they use concave polygons. If you need a concave shape, this can always be achieved by placing multiple shadow caster objects next to each other to compose a concave shape out of convex parts.
Antumbras (beyond where the umbra converges to a point) are not rendered. To avoid the umbra converging to a point on-screen, avoid using shadow casters smaller than the light radius. Stick to large shadow casters and a small light radius.
When using a light radius, avoid placing the light very close to or directly over a shadow caster. Shadows can fail to render correctly in these circumstances.
Maybe you need a bigger light radius?