Im gonna give ya a bit of help with the last guy, hopefully it should help you with them all in some way or other. This just caught my eye to be fair.
Firstly I worked on the palette a tad, pushed the darkest shade to black and the lightest to white for contrast sake tried to have some variation in hue/saturation so it's not so burning on the eyes. I've lowered the saturation of one of the darker shades to add a bit of softness to the skin, plus skin has a reflective quality so that it looks more natural to have some blue's in there.
The trapezius seems far too high on the neck, sort of like a skewed perspective. You need to look at the anatomy of the upper torso as well as the human head construction, it's all connected so it's helpful to imagine what's going on outside the reach of your canvas to determain how to place your shaping and details.
The head seemed a bit too wide, shortened it horizontally a pixel and also lowered the eyes, nose and mouth down a pixel too so it's level to the ears (which I widened). Tried to make the proportions realistic but keep a cartoony quality to this.
The nose I admit is hard to shape in such a small area, the only way to really get around this is to hint whats there and make what you can show easily readable so the eye can make up the rest.
You should use your palette shades to chisel your character into shape more, i've kept in mind where I want the cheek bones to be to help me shape that area, with the light coming from the top left there'd be a degree of shade change from just under the eye to running down to the chin that can really put some shape and interest.
I've used the black to darken and thicken the eyebrows in comparisant to yours, lowered it to the top level of the eye for the frown and increased the shadowing it projects.
Try use the white to highlight more than anything, the odd shine here and there really can help to show that there's light present in the image.
I could go on for ages but lastly i've added what adding hair to the bald guy would look and how to approach his hairline, you really need to know the full construction of the head before going in with adding hair, the image just above needs more work done there because the hair seems to clung to the head. Also added in more detail to the frown between the eyes.
Hope this helps.