From the PDF reference manual.

Design Issues Affecting VF Garment Simulations in Poser
While testing Virtual Fashion garments with Poser, we discovered that certain design choices may either cause very slow simulations or fail simulations altogether. These are the two areas where such problems may occur:
• Narrow strips of fabrics
Some design elements such as bands, straps or sashes may result in an unevenly distributed mesh when exported from Virtual Fashion, and as a result may affect the cloth simulator performance and render results in Poser. Specifically, the Poser cloth simulation may in some cases be degraded, and render results may show shadow artifacts. These effects may occur when straps or bands are created in Virtual Fashion by mirroring and duplicating cuts using the Cutter options. The shoulder straps in the following figure are an example of straps created by mirroring and duplicating an original curved cut.
The resulting mesh may include polygons that appear to be elongated or bunched together, as shown in the following image. When converting such a garment to dynamic cloth, in some cases the cloth simulation performance may be markedly slow or may terminate. In order to improve simulation performance in such cases, the straps can be removed or “constrained” from the cloth simulation.

When I constrain the staps on my Little black Dress they float above the shoulders when I don't they fall through the shoulders at the contact point.