
One reason is due to missing surfaces within the geometry. There are ways in which you can repair the mesh, but if you have a whole surface or surfaces missing then often it is more appropriate to go back and to repair the geometry. To locate where the missing surfaces are, you need to perform a Build Diagnostic (under the Geometry/Repair Geometry menu). Choose a suitable tolerance (1/10 the smallest geometrial feature) and hit Apply. The edges will be colourcoded by how many neighboring surfaces are touching it, red means there are 2 and this is typically what is required. If you see an enclosed set of yellow curves, this would indicate that you have a missing surface. This can be repaired under Geometry/create surface/From Curves option. If there are less than 2 curves or more than 4, another way to make the surface is under Geometry/Repair Geometry/Close hole. You can put as many curves as required in this option. The more complex the surface the harder it will be to create the surface and s.o you might need to break it down into more manageable sections.

