While that one guy with the tall head was intentionally silly, I think I’m struggling most with proportions. I’m using a reference image for most of these, but none of the end results look much like the people in the photos. I’m also not using a grid as I’m doing this all with pen. I know I’ll eventually get it with practice, but is there anything specific I could try?
Try unique reactions next. For example: a grimace, an eyes-closed gut-busting laugh, an angry shout, a Spock eyebrow raise, someone brushing their teeth or about to inhale a big sandwich, blowing a kiss or whistling, etc.
Find a reaction gif or meme that speaks to you and draw it.
Jon Stewart is fun to draw.
Also, try drawing hands, feet, trees, and insects.
I look forward to your future work; you’ve got a great start here!