I'm trying to create the mold for a craft knife I am making for a uni project. But I can't knit the sheets together, I think the problem lies with the shut off surfaces. If i select "all no-fill" and try to then knit the surfaces manually afterward then only the core of the mold does actually knit. I don't see why the surfaces wont knit, and in all honesty I'm getting frustrated with it, any help will be greatly received.