I have found that most of the books that come with your machines today do specify the bobbin type/class needed. And also read the package when buying bobbins - they list the machine types that particular size bobbin is used in. Some of the plastic bobbins are crowned ( a slight hump) in the center and some are prefectly flat. So, do read carefully.
My flat metal ones do work in my machine that takes flat plastic bobbins.