I think the problem was me not really understanding how to use gelatin correctly. I had been trying to follow the beersmith instructions:
but I wasn't getting it fully dissolved - I probably even had mostly a slurry instead of a solution. So, last night I added gelatin again, but this time making sure it was actually dissolved... and less than 24 hrs later, it is looking nearly crystal clear.
For the one that worked earlier, I can only guess it was dumb luck of that one getting more of the dissolved part and less of the slurry part to begin with.