It is tempting, when a project is not moving forward, to chase
after contributors, to ask them for new due dates... which may work, in
time, but at what price? After a few "regular"
reminders, if nothing is really moving forward, then stop.
Observe the situation and decide to act differently.
Get them to take a position
What is the position of each of your contributors in the
project? Is it in the interest of each of them to participate? What do
they expect to contribute? What are their principal obstacles?
Are there paths of action? … It's not your job to tell them "we
have to" but also up to them and/or their supervisors to say "we will".
If the contributors embrace the idea of playing their part in
the project, then it becomes necessary to remove the obstacles that
prevent them from involving themselves as they would wish
(organization, priorities, form and timing of deliverables...)
If they do not embrace the idea, which may be for perfectly legitimate
reasons, then it is preferable to envision replacing
them, even if that creates even more difficulties initially.