How to draw human body in chest/collarbone area?
I think the main reasons the curviness seems lacking is because the contrast is too high or uneven, and your hatching doesn't follow the curves of the body.
The quite abrupt ending of the shading on the breast(s) - the upper part, that seems to be erased with a kneaded eraser in a dotted way, and the dark area of the person's right breast (image 1) - gives the idea there is a sudden concavity or bulge where the transition should be smooth. Curviness, plumpness or fleshiness are all about smooth transitions.
Also look at the curvature and the colour value of the sternum in the original image: it's not as bright as the highlights on the sitter's breast, and there is a gradation from right to left, where it is partially obscured by the right breast.

Image 1
The shadow on the left shoulder has its darkest shade in the contour, close to the light, while the original has a quite even cast shadow, and the rest of the shadow seems flat because it seems completely independent of the shape underneath it (image 1).
I suggest following the lines of the body while hatching, and initially only focusing on the shading that is a direct consequence of the local shapes - the shade underneath the collar bone, the even subtler shade in the depression of the tissue between the arm and breast, and the shade on top of the shoulder (very hard to see here, but compare it to the original):

After these have been shaded satisfactorily, the cast shadow can be drawn on top of it, evenly, so the underlying structure will remain visible.
Treat the entire body as the one single entity it is, and shade accordingly: note the lightest parts (the faces where the light hits at such an angle that it bounces directly towards the observer), and the darkest parts (that are opposite the light source or obscured by other parts or objects), and shade these all evenly.

Some additional remarks:

The sternocleidomastoid muscle seems a little misaligned: in your drawing it seems to originate at the sternum on the left side (of the person in the picture), while it should originate at the right side.
Make it a habit to look at the shape of areas between forms, as they will often give evident clues of how the shapes on both sides are related.
For example:

The shading on the face is very well done, because there is an obvious visual transition between the shaded and lit part of the face.
The shading on the person's right shoulder is also well done, but to emphasize the light hitting it, I suggest making the edge of that well-lit part a lot lighter - unless you plan on making the background darker. Contours are imaginary help lines and don't really occur in real life. They should almost without exception be as light or dark as the shape it is the edge of (they seem to occur in photographs sometimes, but that has to do with the way light is captured and/or the compression of images).
Consider working with charcoal to (partially) circumvent the problem of hatching (depending on the quality/type of charcoal and paper), as smooth transitions will be a lot easier to attain.