Software process improvement (SPI) is a widely recognized approach that software companies implement to improve quality, productivity, and time-to-market. Assessing and analyzing performance improvements are important SPI activities. However, many SPI managers have found it difficult to develop and implement effective performance measurement programs for SPI, in part because guidelines for conducting SPI measurements are scarce. We address this gap in the SPI literature by examining major problems that SPI change agents encounter when developing and implementing SPI measurement programs. We report on a longitudinal study of an SPI change initiative and the challenges that the SPI Team faced in dealing with the issues of measuring effectiveness of the initiative. We systematically analyze an SPI performance measurement program to understand its limitations and the problems that the SPI Team encountered when implementing it. We used an organizational change theory framework to derive theoretical and practical insights that can help managers and researchers develop and implement better SPI performance measurement programs.