Where exactly in main.c have you put "shadow_stencil"?
Have you searched all your code (inclusive all includes you use) for another shadow_stencil command that overrides yours?
GS generates a decal shadow instead of a stencil shadow so there's something wrong somewhere in your code.
Maybe you should break down your code to just have a main.c that loads the level.
Regards,
Pegamode.