Key stakeholders may have little or no interest in a stakeholder process.
Let everyone know who is or isn’t on board. The decision about whether or not to go forward ultimately belongs to the other stakeholders.

The time and money needed for an authentically robust process are not available.
This is actually a fairly common problem. Some conveners or sponsors want a very complex process done in too short a time frame or on the cheap. Provide solid judgment on how long a process might take and how much it might cost.

The political timing of a process seems questionable.
Political or legal deadlines often turbo-charge a collaborative endeavor, but if some legal or political decision is impending, it might influence the nature of the issues. Consider advising prospective sponsors, funders, and stakeholders to wait.

There is fatigue with previous collaborative efforts, either on the particular subject or with other processes.
One way of overcoming this is by reinforcing the idea of “go and no-go” stages in which stakeholders can make collective and/or individual decisions as to whether enough progress is being made to warrant their continuing involvement.