[Chimera-users] surface

Hi chimera users/developers, I have a problem with a surface calculation from one monomer of a structure that I want to show. As you can see in this image http://imageshack.us/photo/my-images/801/testss.png/ it looks like the surface calculation failed (?) because the surface is missing in the middle (note the chewed up structure of the surface). Most of the surface is smooth but for whatever reason not this part of the structure. I have played around with probeRadius, vertex density and zone and they all increase quality but do not add the missing surface part (I'm not even sure if zone is applicable to surfaces at all or only to electron density maps). Any suggestions which parameter I am missing? Thanks for suggestions, Gesa