Cutting off shadows?

I have an odd problem with some of my outdoor maps. I have placed custom doodads with custom models on them, and when I "calculate shadows and save map", the shadows get cut along the large part of the grid.
I would like to know how to deal with this, as the trees/walls/tents that I'm using are in a wintery campaign with loads of snow. I'll post screenies in a moment.

Edit: nvm on the screenies, must be too big or something.