Software ecosystems provide an effective way through which software solutions can be constructed by composing software components, typically applications, developed by internal and external developers on top of a software platform. Third party development increases the potential of a software ecosystem to effectively and quickly respond to context-specific software requirements. The boundary resources model gives a theoretical account for cultivation of third party development premised on the role of platform boundary resources such as application programming interfaces (API). However, from a longitudinal case study of the DHIS2 software ecosystem, this paper observes that no matter how good the boundary resources a software ecosystem provides, third party development remains a mere possibility until there exists adequate external generative capacity. Taking into consideration this observation, this paper makes a contribution by extending the boundary resources model to foreground external generative capacity alongside boundary resources as factors that influence third party development.