is there any explanations why?
CAST is all about stencil shadows, so if stencil shadows were your problem, Helghast's solution would've nailed it.
Decal shadows work differently. If you have a decal assigned, it'll be used, so you simply need to tell the program that you don't want it anymore the say Bart said.
I hope that covers what you were asking.
If the manual's anything to go by, you should also be able to fix it with "reset(my, SHADOW);".
Jibb