Aardvarkphil, you're improving noticeably with your sketches. I think the your faster sketches like your 20 minute figures come out a little closer to life than your longer studies. Perhaps this is because you're spending less time mulling over smaller details, before nailing all your proportions. (I have the same trouble quite often with my drawings and when i'm doing alot of other things. For instance, playing a game of darts, if i think about making a bullseye and how i'm going to throw the dart just so, I'm almost guaranteed to miss.)
Your line weights are really heavy in a few. Some with the pastels have outlines that are a bit to heavy and make the work choppy. Light and quick strokes i think you'll find will help you alot. If you can make this common practice, your drawings will feel alot more relaxed, not just in appearance but also in the sense of freedom you feel when working on them. I would like to add that i really admire your commitment to your strokes. I tend to lift my pencil too much leaving alot of wispy sketchy lines all over the place which take away from my work, while also confounding my rendering later on in the process.
Also, try to work your pictures evenly, not getting too hung up on any single area for too long, i have trouble with this as well, and it's been taking me a long time to break my bad habits. One example of this i would say is your portraits. The man's nostrils and the girls pupils are rendered completely black before you've used your values to define the shape of the rest of their faces. Try to take the shapes of the face through a wider range of light and darks to express volume. (the left edge of the girls face could be made few shades darker than the rest of her cheek and it would enforce the sense of roundness. Most of the rendering as it is is all at the same tonal quality.
I know the inclination to go ahead and make the areas you know are going to be that dark like so, but i find when i do that, the rest of the image seems to flatten out alot and it seems to prove more difficult to render the rest. My advice would be to work your areas quickly building them all equally. Feel free to jump around and work another area as soon as you start to feel hung up in any one spot; and the areas that you're sure of how they will look, i would say go ahead and put some necessary information down, but try not to add those final darks until everything else is closer to completion.
"Today, a young man on acid, realised that all matter is merely energy condensed to a slow vibration. That we are all one consciousness experiencing itself subjectively, there is no such thing as death, life is only a dream, and we are the imagination of ourselves... here's Tom with the weather." - Bill Hicks