I think this topic would fit better in the "Bugs" category.
I am assuming you mean that when you are creating a new synth, Photosynth stops. Depending on the number of images that you have selected, the amount of hard drive space and the amount of RAM that your computer has installed, your processor speed, etc. Photosynth may have quite a lot of work to do.
What is Photosynth doing when it stops? (Creating image tiles, uploading images, finding image features, reconstructing the scene?) How long did you wait after Photosynth appeared to stop making progress?
Also, please confirm that I am on the right path and that you are not talking about loading a synth on the website stopping loading at 31% when you try to view it.
I hope to see you up and running soon.