Besides experience, perhaps it's the style. Try drawing your very own self(body) by using a mirror, or try using google to get great poses from real actors. By paying incredible attention to light and space in these photos, you could go for a super realistic work, unlike the usual cartoonish take. The experience gained from this can then be translated into your passion work by more reliably creating the anatomy and effect you seek. I say this not only because my college drawing classes taught this, but because I still use real life references to this day to try to get the best form and value possible. Take my most recent work, which was a decent attempt at a mostly human body with an anthropomorphic twist. I actually took a picture of myself in the mirror performing that exact pose to use as reference for my drawing. It didn't turn out nearly perfect, and I did improvise along the way, but the ground foundation of that photo to go off of was my saving grace when I got stuck or when something looked "off." This is my two-cents anyway.