Why does the end always go wrong?

I am on the last render stage of my last camera and it wont render. Weel to see it wont render full stop finish is a bit of an exhageration but heres the problem, il start from the top.

1. all my cameras are set up exactly the same to render differnt parts of the church.
2. the occlusion layer is set up exactly the same for every camera.
3. 4 of 5 cameras have successfully rendered the occlusion layer but this last camera will not.
i have tried on all computers at home.
I attempted to render at uni. which worked (but extremely slowly compared to every other render i have done at home. (150 frames over 5 and half hours)) i got home to carry on with the render which began fine to have the same problem at home. all the setting i can find have been set the same way at home as at uni but still not working. I have even tried setting up a new camera to render these frames. this problem is just baffeling. Looks like il have to render on the uni comps if it wil work tomo, unless anyone has any suggestions ?