What Am I Doing Wrong!

Levels can help for gross misalignment, but I have not found them useful for the fine adjustment required for best relative flatness. Plus, it's a multi-step process: first you have to make sure the coin surface is level, then level the camera. But is the place you put the level on the camera parallel with the sensor, and is the sensor parallel with the lens mount? Until I learned the mirror trick I did my leveling by taking pictures of the top surface of a coin wide-open, then looking at the top/bottom/left/right edges and adjusting the camera/bellows until all were in focus. This works, but is tedious.

Of course if you just give up on leveling all together you can compensate by using image stacking. This will ultimately give you a better image because you can shoot at a wider aperture that has higher resolution and gathers more light. Coins are flat enough that usually just a few shots are sufficient to get everything in focus. Problem is there is a lot of work required for each image. But if you are intentionally tilting the coin for lighting purposes or to get a particular view of a variety, stacking may be your only hope to get a good DOF.