I am trying to use the boolean modifier to cut holes inside the long object that is displayed in the picture. The holes need to have the shape of the 4 diamond pieces that or also displayed in the picture. When I'm using the boolean modifier with the option difference selected, it only works properly with 2 of the 4 shapes (as seen in the picture). To my knowledge both the parts are healthy. I already recalculated the normals and removed doubles on both the objects. I also tried to triangulate bigger faces and subdivided the mesh of the diamond shapes but both resulted in worse outcomes.
The bmesh fails in some cases, try the Carve solver instead.
The documentation is very sparse about the inner workings of this option.
Determines what set of algorithms are used to calculate the boolean operation. Carve uses the external Carve Library while BMesh uses Blender’s built-in library and should give better results.