Your work appears flat because everything seems to be lit from the front, there is no clear light source in the piece. Lightning is the tool we use to show forms and sculpt shapes with, when your piece appears uniformly lit everything looks like it's one the same plane.
I quickly doodled over your piece.
I assumed the Dragon would be lit from above so I started blocking out different lightning planes. See how this makes the shapes more believable and readable. Try to pick a clear light source and think about how the forms in your picture would react to it. Sure, that light source could be frontal but that doesn't mean that ''pillow lightning'' is the answer.
Also the way the texture of the dragon doesn't change direction with the forms is also something that flattens it.
Something I didn't touch on in my edit is that the tail seems to be on a different plane than the ground, not resting on it. Partly because it seems to follow another perspective and because of the heavy outlining. Also the lack of shadows ( other than the ladder ) also contributes to the flatness of it. I did a smaller edit of the knight where I lit him from the front and added just a small shadow. Lightning is a wonderful tool to make your image seem more alive.
Also on a different note the ''wing horn'' on the dragon blends into the background a lot. I'd consider changing the color of it.
Finally i just have to say that it is already a very pretty picture and I'm glad that you want to take it even further! I wish I had your productivity!