The only thing I see that's holding back this female from appearing female is your use of lines. You have sharp angles all over the place.
Try drawing her without any straight lines, not as the final piece but just as an experiment. Also, in your last concept piece it seems as though her torso is sprouting from her hips, it has this odd faun/satyr look to it.
I understand that you're going for a tom boy look but most tom-boys are STILL women, what makes them instantly recognizable are their "chosen appearances." A Baseball cap, a hoodie, a short hair cut. They like maintenance free but still girly. You've drawn a man with boobs, there's nothing here that makes her a girl which is your MAIN problem. The tom boy thing is easy, her attitude will tell us that.
Let's focus on what Misj said. Curves!