In draw mode, zoom in on areas where objects are tangential to each other. In some cases where a polygon should align tangentially to another polygon, it does not actually align. Even if there is a very small difference, the mesh generator will fail to create a mesh on this geometry.
There are two ways around this problem:
1) Make the polygon align to the tangent of the other object exactly, but this might require some elaborate trigonometric calculations in some cases.
However, even this type of geometry can result in very bad triangles. For very small gaps, enter "Boundary Mode" and delete the sub-domain borders inside the structure to avoid bad triangles.
2) Misalign the polygon to the circle so that it does not generate this very small geometry gap.
