Also, don't forget that using a Bishop to propose a cease-fire is generally going to have a greater chance of success than using a regular Emissary. This is especially true if the Bishop has a few stars under his belt.

That said, in the end, the strategic AI can be rather short-sighted, and you may not be able to secure a peace with an enemy faction despite everything you do. This seems to be especially true when it comes to the AI recognizing the overall strategic situation vis-a-vis yourself.