Managers and scholars emphasize the importance of inter‐departmental integration to successful product development and emphasize the importance of product development capabilities to firm performance. Interdepartmental integration can increase shared knowledge; this in turn can reduce the severity and incidence of seemingly unnecessary and costly mistakes leading to more efficient and effective product development. This paper examines the sources of shared knowledge problems across functional and product‐based groups. Using interviews, product development plans, and time‐sheet data, we look at the underlying factors behind two sources of knowledge sharing problems: (1) a serial product development process (i.e. little or no face‐to‐face communications) and (2) ineffective use of integrating mechanisms because of sticky knowledge. For each problem source, we find two key causal factors. We discuss the problems, their factors, and the implications for management and theory.