Make sure that the Pixbuf has an alpha channel. And you may have to give some alpha threshold value in render_pixmap_and_mask(threshold).
Pixbufs support 256 levels of transparency while a mask only knows two levels (transparent and opaque).
The threshold determines where the distinction is made between transparent and opaque. The default value that is used when not specifiying a threshold is 127, which means that only pixels below 50% transparency will be transparent in the mask.