Governments and public sector organizations are digitalizing their service provisions to cut costs, improve public administration efficiency, provide better services to citizens and companies, or increase transparency of expenditure and decision-making. Often these attempts are local where each agency, city, municipality, or even a single department develops its own solutions. However, it is difficult to scale these up to broader contexts, e.g., governmental or regional level since they are designed for local needs. From this perspective, top-down initiatives, such as the use of common platforms, may result in more benefits. In this paper, we study a government-initiated platform for citizen-civil servant messaging in eight municipalities, each autonomously making decisions related to their service provision. The municipalities got a shared grant to co-tailor the platform to save resources and to learn together how to make best out of it and avoid possible problems. We study why this joint endeavor turned out to be difficult by utilizing the windows of opportunity theory as a research lens. We identify different challenges in service co-tailoring, including dissimilar practices, processes, skills and competencies, and attitudes and goals, and argue that co-tailoring and co-experimentation necessitates careful planning and consideration of these differences.