I've filled up eight sketchbooks. I finished three of them last year lol. Ho many sketchbooks have you all filled up?
The muscles are the problem. The arm muscles are very confusing, specially near the shoulder. I've been getting the hang of it tough. The stick figure thing is something I do for all my sketches.
I would look at work by other people, which you may or may not have already done. I use the Walking Dead comic books, specifically the first volume by Tony Moore.
I look at Comics and pictures showing the human muscles like this one: What I'll do is fill an entire notebook with them, that helped me with the shoulders.
The bad part is I'm an 'artist' since I was 3 and none. I have a portfolio full of drawings and paintings, but none in a sketchbook xD I guess sketchbooks aren't my thing
let's see, well I have one for doodling and one for serious drawings, i've filled up about 4 200 page sketchbooks so far c: