typically, the bobbin thread used when embrodiering is finner then the bobbin thread you use when piecing or sewing. So there is less resistance in the bobbin case so that means the bobbin thread can show up on the top. It all has to do with the balance between the top and bobbin threads. Normally when embroidering we want the top thread to be loser so that it is pulled further down into the fabric so we don't see the bobbin threads on top. you might be able to adjust this by tightening the screw on the bobbin but most experts recommend against doing that as then you have to remember to loosen the screw again when doing regular sewing and sometimes it is hard to get it the right spot. So most experts recommend that you purchase a second bobbin case to use when doing something that requires a different bobbin tension such as embroidery or in the opposite direction bobbin work which would require a thicker thread in the bobbin and hence needing to loosen the tension screw....in short....I guess what I am trying to say, purchasing a separate bobbin just for doing embroidery makes sense and now sewing machine manufacturers are making bobbins specifically for embroidery....I have a Bernina and I have purchased the "yellow" bobbin case for embroidery....I haven't tried it yet...