Tree shadow problems

[Greg Law]

Sorry I put this in the wrong forum this morning so have repeated it here.

I'm having a problem with my Vray pluggin render. My trees have stopped rendering the alpha cutout in the shadows. I'm getting the shadow of the card not the tree. Does anyone know what the issue here is. I was wondering if it is file size. These are large files. 65meg plus.
My machine is an
i7 950
12g ram
Quadro 4000 card
I know the ram is limited by the 32bit program.
I'm using the default Vray render settings at the moment.
The tree file types are PNG format.