When you're using curvature as a mask, make sure that base, concave and convex colours are fully black or white. Usually you want to keep base full as black and either convex or concave (sometimes both) as white, that depends on your setup and what you want to achieve. Next you may need to check max distance - depending on your scene units setup and your object size, you may need to increase or decrease its value to get meaningful result. Then it's a matter of playing with ray directionality and colour spread. Launch interactive rendering with isolated object of interest to have best responsiveness, adjust one control at the time and watch how it responds in render. It's a good practice to temporarily replace base and secondary materials with some simple and bold colours to get better visual feedback while adjusting curvature mask.
If you feel that you still have hard time to control curvature, you may want to give a chance to Corona AO map, it's very similar to curvature, but since it was there for much longer, you may be more familiar with it and get good results more intuitively.