<stands up> Hi, my name is LadyPengwern, and I am a fangirl.
Been looking for a decent pic of Ventris since I read the Graham McNeill Ultramarines Trilogy years ago. This is so close as to be a lot like I imagined. Love it! Love the other goings on in the pic too.
I really like the coloring here, and how you've composed a moving scene - it looks like the marine is kneeling among the bodies of the fallen, and I like how the personality shows through. Maybe it's a little flat, like you said, but I still think it's a great picture.
As for the artwork being flat, it may be that you've given an equal value of lighting to the foreground, midground and background - sometimes that can really reduce the depth of a piece. Try messing around with line-weight as well for that illusion of distance. Hope that helps!