If you have even a tiny area that is flat and not properly domed, it will not take a proper polish. You have to be patience and learn to get the entire stone evenly domed. Look at the stone from both ends and both sides. The curvature should be even in every direction and all the way to the center. You should make this inspection at every step, but especially in the first, coarsest stage.
Any irregularities are easier to correct now than later with the finer grits.
Now we are going to get ahead of ourselves for a bit, just so you know where we are headed. As you are shaping the gem, leave a small, vertical area on the sides before it begins to taper in. You will also want to sand a very small bevel on the lower edge of your girdle. This is done in the fine sanding stage. Cabs are usually placed in a bezel setting.