probably your mesh has some problems, try cleaning its history, and then go to Mesh -> Cleanup, and clean the mesh up, you can try all of the settings up, then try smoothD, but some of the settings might change some geometry, (like Remove Geometry Nonmanifold geometry, Lamina Faces) if you have a good result you can try to uncheck some of the these settings, i once had a similar problem and i believe it worked that way. But if the problem is really bad geometry, i advise you to cleane it up and then work on the changed geometry.