What kind of cameras are they? If they are 36mm with manual settings, it may be the speed of the film isn't working in your favor. Did you use your flash? A combination of the film speed, camera settings and use/not using a flash could all contribute to getting black pictures. If they are digital cameras, what settings were you using?
I have done a lot of night photography of wild animals, and sometimes we even had to use a slave strobe and reflectors to help cast more light on the area. Just my thoughts... =) I am sorry your pictures didn't turn out.
It reminds me of my visit to a monastery in west Texas. (I wish I could remember the name of it!!) They had a weeping icon of the Virgin Mary in a very small chapel. I asked the monk if it was alright to photograph it, and he said "Certainly, if you would like to try", with a sweet, but rather mischievious grin on his face. I took aim with my Nikon, and I could NOT get it to click!!!!! I tried again, and nothing happened I went outdoors a few minutes later, and took a pictures as if nothing had ever happened. The camera has worked flawlessly ever since. The monk then told me that the icon of Mary often causes cameras not to function. That was an experience I will never forget. It can be explained in a religious context, or in a paranormal context, and it still leaves us feeling that a force was at work there that we humans don't fully understand. I know that I felt strangely blessed to have experienced something like this, but I guess this is a common occurrance with that icon.