There seems to be a bit of a disparity between some of your works. The second, third, and fifth drawings have very particular and clean details, whereas the first and the fourth are a lot more flat and seem to have some minor anatomical quirks. My first impression is that they looked traced (which is not a bad thing! It's great for practice, just gotta ensure that credit is noted for the traced original).
Whatever you are doing for those three in particular should be used as the foundation for development. If it's tracing, migrate to just using the reference image to sketch out the bodies form. Think stick figure, not body outlines. Just sketch in the length of limbs and positions. Then use the reference image to try and complete the rest yourself without tracing.
If for those three, you were using just visual references or some other form of guide, it had a great outcome and you should continue down that path. Still life is also a good practice to get more comfortable understanding shading and depth. Also, this site might be useful for you- It's nice for getting that basic stick-figure form to go off from: http://www.eggazyoutatsu.net/eng/atarichanDrawer.html
Just keep practicing. :3