I'm coming in late, but I have a few comments. First, I agree with the suggestions about looser cropping. In some cases the tight cropping works (e.g. Land18 and 11), but in other cases it just feels cramped to me (e.g. Land25). Second, for the long term you should think about upgrading your lighting equipment. As I recall, you're using mixed lighting, including long fluorescent bulbs. This combination causes a couple of less than ideal effects. It produces long catch-lights (highlights in the eyes), which I find distracting (e.g. in Land30 or Land35). It also gives you mixed colors of light, e.g. in Land34, where the two sides of each face are (slightly) different colors. This is usually not what you want in a portrait.
Finally, some of the black and white conversions have problems with skin tones. I think the reason it's happening is that you have flat (low contrast) lighting with highly saturated color, along with the mixed color lighting. Whatever the cause, you have lots of colors in the face, and these get translated into B&W as tone gradations that do not correspond to the shape of the face, i.e. do not correspond to highlights and shadows caused by shape. Instead, the skin ends up looking a little mottled (e.g. Land19). Given these color captures, I would use heavy red filtration in converting to B&W, and reduce the contrast and increase the brightness in the final image as well. But I am no expert.
I'm not trying to be critical, by the way -- just trying to provide some technical suggestions, based on little knowledge. You have some fine shots in there, and have done an excellent job posing the subjects and making them comfortable.
