We see this sometimes when re-uploading the same photos, and think it might happen when a previous synth was canceled or we had a problem with our image servers.
I'll let the team know we're seeing this again.
What you can do: "touch" all of your photos by adding a tag to all of them. This is easy to do with Windows Live Photo Gallery. Then, create the synth again. This will force all the photos to get uploaded again and the new synth should work.