hiya, i think the issue may be with the thickness of your pieces. the plastic is printed layer by layer and its done. the brass however is printed in wax first then covered in essentially cement, and i think at this point it is warping due to the thin "soft" wax material. so then you get this warping when it is cast. Try thickening the walls by about 0.2-0.4mm all round. It worked on my snitch. It may make it more expensive but you would get a consistant quality then.