your stroke is good - it might need a bit more fine tuning in terms of a cleaner image. ( but they are sketches, - so no real need ) - i think you may play a bit more with different pressure of the pencil. - you still "frame" the shapes- and the frame doens t extend into the objects ( shade etc. ) - but again, its only sketches. ( thats just a brief examination of the last 2 images )