Get the management and officers skilled correctly. All should have proven experience of management and motivation of volunteers (and if a gap, provide specific training for this). They should have KPIs (key performance indicators) regarding level of face to face or direct phone engagement with their volunteers, as well as KPIs around volunteer feedback and retention. Merged region or roll staff should have clear time ring-fenced and available for each locale and service.
Minutes for meetings with tracable agreed actions (action, to be taken by whom and by when).
Look at training and communications. Each team should have clear communucation plans (what's communicated, by whom, to whom, when and how) and a 6 or 12 month training schedule so volunteers can try and align with the skills development elements applicable to their needs.