very 3d!

One thing that I'm noticing is that your lines are extremely thick. If you could practice doing your light construction lines with either a mechanical pencil or a pen, that could help immensely with that losing lines in overlapping forms problem. Or at least, it's helped for me lul. I use the Staedtler fine liner pens. But many things work. You're doing great with these figures. To avoid getting confused by the knees, just take the extra time before committing to a spot to observe where they're set in, and what direction they're aiming for, you're already doing this with your boxes, so just observing where those points are aiming for is key. I notice Stan a lot of the time before committing to drawing a large form, will often draw directional lines to indicate and remind him of what the perspective is and where they're going. Hope this helps!

I'd like to suggest working on your line quality and trying to be more controlled because I think you're a little unsure of yourself in places. This also helps with communication for yourself and others. When you take your mannequinised form and turn it into a finished drawing it helps to be clear on communicating what you're trying to do. You don't want to get confused part way through asking yourself "Is this leg or arm going away from me? Towards me?" I'd work on your proportions as well cause in some of these images they have small heads, big butts large legs and small arms. This will come more with time and praticing more and more but praticing proportion and perspective will help with that. I'd like to suggest practicing drawing boxes and cylinders. Here's a couple of examples. Grab some paper and draw the cube and cylinder rotating in space, this did me a world of good when mannequinising cause it meant I can picture the hip ribcage and pelvis as a box or cylinder in space on the body. Lastly and this might not help but it's how I like to think of the body when mannequinising it. I think of it like firm tough rubber. It's tough and capable of being flexable but not to the point of breaking. If you go to far with bending then you've broken the figure. Say a character has a strong S gusture in their leg you can carry that over in the mannequin and help maintain the character, action and the weight. The downside is you loose some gesture but that's why it's important to exaggerate your gesture. TL;DR 1) Work on your line quality practice holding a pencil and try to make nice clean lines. This will help in communication. 2) Practice Proportions and perspective, this will help make the figures feel more 3D and help depth. Try studying the cube and cylinder to with manniquinisation. 3) Think of the manniquin as firm rubber, if you see a strong S or C curve in the gesture try to carry it over cause it will help maintain some of the character and the weight and action.

That's a good start! I would suggest you ghost your lines (make the motion with the pencil without touching the paper) before laying them down in a single stroke. At the moment, they lack confidence.

What helps me with knees is thinking about how the hard blocky shape emerges from underneath all the soft muscle folds versus an "end cap". It's easier to see with people who are super jacked, but the principle applies to everyone.