Hi Micah,
There are several ways. First, you can angle the chisel up a tiny bit which will make the cut go in deeper. However, you have to be careful that you don't bury the graver too deeply so that it cannot cut any further. Be gentle and gradually deepen a cut. Another way is to cut the line again to deepen and widen it. That is probably the best solution. A third option, which is useful for scrolls and curves, is to simply angle the graver over on its side slightly as you cut. That will widen the line. I also use gravers ground at different angles to get different line widths. For example, with fine detail and shading I use a graver that has a 90 degree cross section. However, I often use a graver with a 110 degree cross section for borders where I want a thicker line.
dave