Please upload your art at 1x, don't pre-zoom. The forum has its own zoom feature (click to zoom in, click with any modifier key to zoom out), and it's easier for people to make edits if your art is at 1x.
I think your dithering on the shell isn't really helping to make the shadows look soft, since the colours are already low-contrast enough to produce a smooth look. Instead, the dithering just looks noisy. Without dithering, you would have more space to communicate the exact form of the shell, and maybe even some details such as ridges.
It's good that you're shading the shell based on its overall form rather than the details on it, but your shadows form a flat gradient and don't communicate the rounded form of the shell.
I tried to edit the shading on the shell to show the form of it a little better without dithering, I hope it'll help:
Since you know what the shell's form is meant to be, you could probably do better at communicating it than I did. If you're not sure, I recommend looking at reference of real snails. Even though your snail is quite cartoony, it's realistic enough that you could learn a lot from looking at real ones.
I also tweaked the colours to make them a bit less dull by adding hue-shifting (bluer shadows, warmer highlights in this case), to get rid of some redundant colours, and to make the shadow on the slug's body more visible. I also got rid of the double outline on the shell, as I think it made the lines look blocky.
