Heyy. This was a very cool painting, and I really like it! So, I saw this thread a few days ago and I didn't really want to touch on the matter, seeing that you were getting so much help. And you did receive a lot of help, not saying that the help was bad. However, I noticed a running theme in the paintovers you got, and it was mainly about contrast and and odd saturation. And interestingly, the fog/dust behind the dragon, was always removed in the paintovers above. And I thought that was kind of sad, for although it indeed did contribute to making things flag, they are kind of taking away from the piece by doing so. Now, I have taken some time to properly show a few of the key elements that made this look weird. I also avoided trying to distort the image, change the character or the dragon. And this becomes important when I'll address form.
So firstly, the main issue with your painting is color. Sit back, and look at it. It is nice, but you start seeing the same color over and over again. In fact, most of this is yellow, and what isn't yellow, is very desaturated. The weapon sheath / cloth is more saturated than the dragon! Dragons are high fantasy, right? Make that thing look fantastical!! 
So, color is an issue because it limits how you can create shapes and nice forms to some degree. Color also makes things more interesting, and are incredibly important when thinking about composition. And in this case, your colors weren't playing at composition at all. The exception is the weapon sheath, which stands out very much and brings attention to the sword. Though, in my opinion, the sword should've been closer to the character or something, as it is too far from where things are happening. So, I made the dragon look a bit more fabulous, changed the background color to a blue. Now, you might call me a hypocrite because I'm "taking away from the piece" by doing so. However, I did have a reason for this.
The bright light that the character is shining on the drakes face. This was almost lost in the original piece, and it wasn't only because of contrast. It was because of the color!
So, second point. Values. You have already taken a look at values by turning off the colors, so I don't have to say that much. However, I do have a few tips. In my paintover above, I didn't simply crank up the contrast and called it a day. I went over and tried to define some of the facial shapes more, and gave them a greater variation of both colors and values, enriching the shapes. In your original painting, things were quite tame, as the brightest lights on your drake were the same over the entire drake. So, here is where I changed yet another thing in the piece. You have some bounce light under the drakes head, and I mostly removed that because of one thing. Chiaroscuro. What is that, you ask? It is the technique used in the renaissance, by great artists. It was a technique of values, of shadow and light. This was a great time of discovery, where many things in art were discovered. Perspective, anatomy, and Chiaroscuro brought drama to the table. Yes, back then, everything was dramatic, but it worked. And I thought that your scene was a bit too colorless and simply cold, to represent what you were trying to show. A dramatic, or interesting moment. Why do I keep repeating the word dramatic? Because the entire technique was about having heavy heavy darks and having some light. Now, I haven't totally converted this piece to that, because, it'd change the entire artwork, so I hinted it, so it would at least create some mood.
So, values. Values are about form, and I think I understand why the other paintovers were mainly contrast adjustments. The forms in your painting are kind of undefined. You spent a lot of time on the eyes and cheek of the dragon, making it look like something. But the mouth area is almost without detail, and this makes it hard for us to make it look less flat. Of course, it doesn't stop us, after all, a ball still looks 3D. However, it makes it kind of boring, as there will be less interaction of light.
These were the main problems with the flatness. I decided to address that, directly, as this is kind of a big scene, and you wanted us to help you the "flatness" and nothing more. So I don't want to get into composition, anatomy, character design and such. Anyway, I hope this was helpful to some degree. This may be a few days later than you had hoped, but I wanted to reply seeing as I didn't completely agree with some of the other paintovers. They had some good points, don't get me wrong! Anyway, I really like your painting, so I'm sorry if I sounded rude in my points above!
