Bonjour Kris,
I don't know if this will help you (my experience with XSI is minimal, and I never done network rendering with it). But I use Softimage 3D in production and found couple of things.
1-I'm only able to use the rayhost file through the interface. When I want to render with a shell, the render does something similar to you: it starts, but does not get further than processing the first frame.
2-Even when I render through the UI, I take a risk. Sometimes it renders only the first frame.
3-No matter if I render through the GUI or the shell, I can't stop the render. Technically I can, but if I do so some parts of the projects (it can be an effect, a picture, or anything else) end up corrupted.
The best solution to this, imho, is to open a shell on each machine and start the render on each of them. I don't know if this can be done with XSI (still strugling with modeling and texturing).
Hope this helps
Salutations - Cheers
Bernard Lebel