Most project managers identify influence, negotiation, political, and sales skills as lacking and most needing development. These skills are necessary to create integrated outcomes which is a resulting skill that project management seeks to achieve. If any of these skills are not present, moving in the wrong direction, and/or stationary, outputs become very limited. Failed projects result.
In order to develop these skills, read The Complete Project Manager: Integrating People, Organizational, and Technical Skills and participate in PMI SeminarsWorld led by Randall Englund and Alfonso Bucero.