When you set up a "promote builds when..." criterion of "When the following downstream projects build successfully", it will not promote if any of the specified downstream builds are successful but unstable. This is, at least for this user, counterintuitive, as the downstream projects did build successfully.

Therefore, I would suggest adding an additional checkbox for "Promote even if downstream builds are unstable", similar to the check box in the post-build action of "build another project" - this would both clarify the actual promotion behavior, and allow promotion to trigger off of unstable (but successful) downstream builds.