One of the most difficult things during the project is to manage stakeholders who are also co-owners of different parts of the project.
If done well, the project is running smoothly and without delays. This is, unfortunately, rarely the case.
Now, to the point:
Sometimes you have to be extremely clear if you want the project to go without delays. Other times, you just need to present the ultimatum in a clear and simple way. I like to do it with a touch of humor, this is why I use the rule qui tacet consentire videtur, also known as he, who is silent is taken to agree, in most emails to decision makers related to the project.
Wait. Stop. What? Decision makers?
Well, sometimes the given is, that people should be validating different streps and processes are unresponsive. This is when you, project manager, have to think about the decision to be made. My internal workflow goes as follows:
Dear John Doe,
As you know, I need to send the composition to be printed today if we want to have the billboard ready before Mondays conference.
I still don't have the feedback from you.
If you don't reply till tomorrow 11 AM, qui tacet consentire videtur will be applied.
Have a great rest of the day!
Now, taking the decision without the actual agreement of the key decision maker may be difficult, and I do not encourage you to apply it unless you are sure about your position within the company and the amount of decision making power you have been granted. In most situations though, personal fears in a professional level are the reasons why projects are not going further: someone is being a bottleneck or just gets unresponsive and then you are the one who is not getting things done. How about taking the right decision yourself? In many cases this is gonna be the best you can do.
If you're a PM, lead the project. Worked for me so far, but then, you know what's best.