I had some thoughts about your paintings, Vo, and took a few moments to make some notations in photoshop.

I think the largest issue here is one of lighting. The image has no real light source and, consequently, lives in realm between 2D and 3D. Examination of the details - individual muscles, etc - reveals a suggestion of 3D that works in a convincing way, but stepping back and taking in the entire image has a very different impact, one that is flat. My suggestion is to establish a light direction to give your image an overall 3D effect, as well as enhance drama through contrast.

In addition to this large suggestion, I've made 4 changes to smaller, more technical issues:

1. No one has ever accused me of knowing my anatomy, but the muscle on the thigh here seems too large (although my version may be a too small).
2. The abs are too chiselled. She's wearing a bathing suit, but even if she weren't they'd still have that problem of looking a bit like they're carved into a totem pole rather than a naturalistic feeling of skin over muscle.
3. The transition of the bosom is too extreme, like spheres glued onto a surface.
4. The left armpit is too low, she's tilting in the opposite direction as her armpits.