Check your perspective on some of these! I did a quick paintover of the Samuel L Jackson one to illustrate. For poses with a more difficult angle, check the 3 major axes. It can be helpful to construct a small box to make sure you understand the angles. See this post for more detail: https://www.proko.com/community/topics/i-need-critique-on-perspective-and-structure-of-drawing-the-heads#kU

Keep practising like this

Hey there! Great work so far! Your structure is pretty good but I feel like you are missing the core shadows. Just with the core shadows and the ambient occlusion you can create pretty convincing forms so I highly recommend focusing on them a bit more. Keep up the good work! :)

These are looking amazing! How do you do this without laying down a lot of construction lines first? If it was me I would probably sketch so heavily first and then the final drawing is not very readable amongst the reference lines. How do you do that?

Looking good, in addition to using more of a value range, I suggest you vary your lines for a more interesting piece, for instance using soft lines for soft hair.

They look really great, I especially like how you draw very characteristic features! The only thing I would mention is that I think you can be bolder with shadows, using more of the whole value range. I think that would make the faces even stronger and even more interesting.

The proportions of both of the faces in the first photo seems a little bit off. 1) On the first face the mouth seems a little bit too high / close to the nose. On the second face the nose and (our left) side of the face seem to be drifting too far left. 2) In the second photo the eyes don't seem to really follow the perspective of the face. The eye on our right looks like it drifted up too high. That being said I think all the other ones are really great. Amazing progress if you've only been at it for 6 months. You really show strong construction of the forms in the 3rd photo. Honestly, I think the issues with the first couple of faces may just be things you already know how to fix and as you get more practice these variances will go away. Just keep on practicing the way you are. It looks like really good progress for only 6 months.

Wow! That looks great!