Too often digital art looks digital. All stroke weights are the same, there's no nuance to the line vocabulary, there's no variation in depth perception, no inherent "wonkiness" to curves, etc. All these things are natural, common, expected, occurrences with hand drawn artwork.
This is a primary reason I have always disliked "sketching" digitally. There's no natural "tone" to anything, everything comes across as digital. Even if "natural" brushes or tools are used. Software has constraints it must live by and it tries to "think" ahead or anticipate what a line/path should look like. Software, while mostly accurate, does make some decisions.
So to overcome this you need to look at your sketches with a critical eye. Pick out what specifically makes your sketch work. i.e. that line is bolder, that arc is not a perfect circle arc, that line has varying thickness to it, that stroke should taper and end in a point not squared off or rounded, etc. Then replicate all this in the digital version, which can be more difficult than it sounds.
My processes is generally a 4 step process...
- Loose sketches
- Refined hand drawn comprehensive
- Scan comp (or digitally photograph)
- Manually trace scan using software
Refining a sketch with pen on paper allows me to be far more thoughtful about line weights, end caps, arcs, etc, while not being hindered by software. I draw something as crips as I would if there were no such thing as software and scanning. This way I don't have to think about "how" to make something look like I want, I just draw it. While often I could certainly open a new Illustrator document and create the same thing I've sketched, by scanning and manually tracing the comp, I can more closely get the nuances in the sketch. Then refine further.