Hi !
The main problem I can see in your sketches is the lack of proportions.
It seems to me that you're looking too much at you're drawing and not enough at the picture. Those kind of exercises engages the look, and you should look out of the picture reference only to position the start of a stroke and immediately get back to looking at the reference.
And when looking at the reference, you may compare the sizes of the body parts (head, shoulders, hands, pelvis, etc...) to adjust the proportions, and also pay attention to the voids around the body.
Also, I don't think in 30 seconds you can get the time to capture facial expression. Maybe try longer time lengths to do so.
Hope it helps you and good luck !
