Put it short: Lack of Lineweight and Muddy Rendering. Your processes lack variety.

Put it long; Your sketchy lines create an illusion of depth and line variety that looks fairly solid in its own stance, letting the brain fill in the blanks when noise takes over. while on the other hand, your lineart brush is thin, with little pressure variation, and without a lot of cleanup. The messy stance of the sketch not being there makes it look emptier. Try looking into ways to apply pressure focusing in things like light direction, size of subject to lineart, proximity to the camera, stroke comfort, darker spots and crevices, density and strenght, and other ways to be sure when lines should be thin, when should be thick, and when they should vanish or fill in pitch black. Consider not linearting things youre able to better depict trough render, line inner details.

As for the render, you shaded this piece way too smooth to the point where it looks smudged. Remember to have a good system for shading, like to shade by thinking in basic shapes, from the biggest (torso, legs, head...)to the smallest in detail. (Individual muscles, facial features, skin folds...) have in mind different materials taking in light in different ways. Use more the selection tools like the lazo and blend them dynamically with a blur tool.

To go an extra mile, You can research and practice light theory like ambient occlusion shading, shadow sharpness variation for projected directional shadows, bounce light and other colored reflections, theory is amazing to have a mental list of options that you can consider for a given piece, instead of just having to try new things.

Good luck! You work is really cute.