Hey, I think keeping up the consistency in value will bring it up a step. Like on the prism you can see the value goes lower on the edges of the shadow face and there are splotches of lower value towards the middle. Also the rim light should never be as bright as your highlights! Your brightest shadows shouldn't be the same or lower in value compared to any of your lights. You need a fine light between your lights and darks. Also I have a little screenshot from one of the proko videos to show you what that looks like Keep it up and you will have it down in no time :)

Great looking drawing. I particularly like the clothes and the shadow shapes; it feels like the fabric has some real volume and weight. Overall, I agree with @Antti Kallinen. But, I think the overall lack of light may also be because of the photograph itself and where you took the it. If so, then try to make sure you have good lighting when you photograph, and, if that's not possible, I believe Proko suggested to photograph in the shade outside in daylight. You can also try tweaking the brightness / contrast digitally if the photograph feels dark or too light. Either way, good work! :)

Well i think your shadow shapes look ok, but it lacks in lighter areas. So either deepen the dark end and make a middletone for the skin, or make a lighter tone. But before that i think you should fix the shoulder area and face if you still can. Thats what i would try. Its got a nice feeling to it, worth fixing.