One thing to think about is that retention is a metric of strong engagement but is not engagement itself. Thinking about activities that will build trust, increase transparency and increase participation in the process of research should lead to increase retention as a metric of success. Have you had a focus group with stakeholders asking why they are not continuing to participate? What about putting a survey into the field asking why some have never come to a CAB meeting or town hall etc. This is a first step in helping you develop mechanisms that may be useful in retaining stakeholders.