I am an art student myself so between high school and college,I've had my fair share of figure studies.
Something that I dont think the other two people pointed out ( I agree with everything they say, they gave you some great advice) is motion lines. You should try to make them your basis for every drawing ,If your going to do a quick 5 min drawing or even a more lengthy 2 hour drawing. But basically what you do is create one sweaping line to get the idea of the motion or the angle of the body.( this line should take you no more than 2 seconds draw, very quick , instictive and to the point) then block in simple shapes,( oval for head, triangle for torso...etc) dont go straight for details, even if it doesnt quite share a likeness to your model , this will help you a lot. I've always found that doing a few quick gestural sketches before I do a lengthy drawing helps me . If your having trouble blocking out simple shapes, sometimes squinting helps you not focus so much on .."ok thats the models head, thats their arm and thats what it should look like) You want to make sure you have correct proportions down before you get into details.
I cant stress proportions enough either. I think its 8 heads ( including the figures real head ) equal their height. Also In one drawing I think you might have had issues with fore shortening. ( its the one that you said you had issues with tone) In your drawing the models head is much larger than their feet even though the feet are closer than you. The opposite should be true in this case. I have always found that the drawing horses/ easels that you work on are sloaped, so they will tend to skew everything your doing, just be aware of this and step back every so often like the commenter above me said. You'll spot mistakes so much easier.
Like everyone else , I dont mean this in a harsh way just some helpful constructive critisim. Your drawings have great potential!