Quick update: thrusters are definitely coming along. I think a major problem was flexibility, which was causing me to hold the bar slightly in front of my body instead of directly over my head. I've been working on form and flexibility, and I'm already seeing noticeable improvements (though it may be the dreaded placebo effect
I'm also making sure to "lock" my shoulders at the top, as previously suggested.
Thanks again for your (collective) advice.