You are having difficulty getting a sponsor for your project
So you've got a project that you think is important but you're having a difficult time convincing potential sponsors that the problem is significant enough that they should care and take action. It could be that the problem is truly legitimate but it's not a high enough priority to warrant immediate action. Then again, it also could mean that you've got a poorly defined mission statement that isn't compelling enough to take action.
The project team is confused about what problem the project is trying to address
I've seen more than one project where the project team goes through the project with different views about what problem needs to be solved. If each of your project team members is unable to consistently recite your mission statement, you're sure to have points of confusion throughout your project.
It is difficult to keep the project team focused on solving the problem
On some projects, I have encountered situations where project team members stray from solving the root problem to solving a "problem du jour" which may or may not be related to the project. At times, there could be validity to the issue being raised, and it might help you to further articulate your problem statement and resulting solution. At other times, though, it could just be a red herring that dilutes your focus and creates confusion about what you're trying to accomplish.