Yeah that's basically it. The thing about light hitting form is that it doesn't create a lot of value variation where it hits. Light is relatively fairly flat and exponentially looses "light" as form turns away. So if something horizontal is getting 100% of the light, something that is at a 45 degree angle isn't getting 50% of the light, it's actually getting quite a bit more. Check out this Sargent painting
http://www.wikiart.org/en/john-singer-sa...n=referral
If you squint, it's not a lot of value variation in the light. Most of it happens at the edge where light turns away into shadow. Now contrast this with the last portrait on the previous sketchbook page. The values change sort of everywhere. A good example you may have experienced would be if you've ever rendered some portrait a lot and then suddenly it looks like their skin is made out of metal. This is usually because the unity and integrity of the light (and it's values) has been compromised in order to push more and more form, making it "too much".
Another example would be the lips of this last girl you posted. There's a lot of value variation around her upper lip and I believe that there would be some, but it looks like you've pushed the values too far so they look over-modelled. The way we see stuff is by comparing contrasts and if you say the there is a lot of variation on something that is relatively flat, you have less "umph!" when you really want to turn the form through value variation because if a slight variation in form changes the value a medium amount, when you then chooses to vary it a lot, it's less impressive. So a simple way of looking at it is that if something is over-moddeled through value, all other value variation becomes "meh" in comparison.
Also, check out this Edelfelt painting
https://www.flickr.com/photos/gandalfsga...6946719704 . Download the full size, open it up in photoshop and just zoom in and look at the edges of the contour and the edges of where light turns into shadow. This is really the place where we see a close study of values and shapes. The middle of forms don't really have a lot of value variation it's at the edges that we see that good stuff ;)
There is also some value compression stuff I could get into but I've already taken up enough space in your sketchbook with this very long text. The TL;DR of it would be to make your darkest dark and dark mid-tones closer together on the value scale so you have more room to play with the values in the important areas of your painting. This would simultaneously allow you to introduce more contrast in the subject's mid and light tones but it wouldn't look over modelled because the darks are compressed so you've established relatively more room to play with. The simple rule of thumb to go by is to push your darks and lights closer together to allow for better value mapping (my profile pic is an example of this taken to the extreme where both sides of the value scale are heavily compressed, there's a bigger pic in my SB if you're interested)
These are still good paintings, man. I like your work, just a little tweaking and you're gonna make some great stuff!