I have a very simple model... basically just a shape pulled to 2 mm thick. Has some cut outs and such, nothing less than 1mm. Simple, flat, nothing tricky. For some reason, Stainless Steel doesn't show up as an option. I have other much more complicated model, some of which have actually be rejected after being sent for printing for having walls too thin or things like that.... those DO have stainless as an option. Any idea why it wouldn't show up? I made succeedingly more simple versions, and even with almost no detail, it still won't show up as an option.