One of the things that you can do to avoid this problem is from the very beginning make sure that you have institutional support, and that your project is built into the goals and missions of the institution. This ensures that the directors, board of directors and other staff members are involved in some way. You can't move forward very fast unless you have many people supporting your project. Try to create projects that do involve all or most of the staff and volunteers of your organization. This is especially important for very small all volunteer organizations.
Community projects are incredibly time-consuming.
Any time you collaborate with other organizations or other groups, you need to add in extra time, because talking takes time, convincing people takes time, and finding common goals takes time. Make sure you build in extra time, and make sure that you build in support of colleagues in your own institution, and that the project that you develop meets a number of goals of that institution. If you work in a historical society and you do public programming and manage collections, make sure that your project meets both goals of doing public programming and building collections. Finally, don't forget that good projects act as catalysts for other projects. Your collaborative documentation project will spawn a host of other activities, both at your own institution and others.