I really do wish that you could re-upload your first attempt so that the rest of us could take a look. It would help to be able to see what you're talking about when you say that "the images keep disappearing.".
As to your question about the 'synthy' percentage, what that number indicates is how many of your photos were able to be connected to each other. If you enter a photosynth's 2D view (sometimes called 'Grid View') you will see the clusters of photos that matched each other. If there were any photos that didn't match any other photos (commonly called orphan photos), they will be the smallest group at the very bottom of 2D view. You can use the tilde key [~] to toggle between 3D and 2D view.
In a 75% synthy photosynth, moving from one photo to the next within that largest cluster of 75% of your photos that did connect will work fine, but when you move to the end of that cluster and off to some of the smaller clusters, you'll just get a crossfade.