there's too much softness and not enough darkness to distinguish itself from the light.
There is this overall kind of flatness to the drawings, which, for this purpose I don't think is necessarily a bad thing. A way that you could make it look less flat is by pushing the contrast on your lighting, possibly.
Basically, this is the problem. Your pictures are getting more out of the light than the shadows. But a good picture relies on both light and shadows to show depth. Just for the heck of it, I threw it into GIMP, and messed with a few settings. Each of the following images took only a few seconds to change.
With lower brightness and more saturation:
With just lower brightness:
With lower brightness and higher contrast:
The original:
I'm assuming you wanted to go for a more "winter" look, initially, so maybe you'd want to use less saturation. But my point is, you can get substantial differences from tweaking just a few settings, and you really
don't have to change anything about the way you're drawing it.
By the way, do consider using PNG format for your images instead of JPG. With PNG, you don't get the image compression, so you won't have stray pixels reducing the quality of the image.
Hope I helped!