I was embroidering a tee-shirt recently with an open design (with light cut-away stabilizer and adhesive spray to anchor the shirt). I was happy to see that there was very little puckering.
Then I had to reload the bobbin. The second panel (it was one of those repositionable hoops) had results much more puckery.
I've done a few since. The only difference was that, for the first, I'd used Madeira rayon thread and Madeira bobbin thread. The second, same Madeira rayon thread, but I grabbed a spool of Janome bobbin thread. I'm not slamming Janome bobbin thread, and it has worked fine for me in most instances, but in this case, it really made a difference in the results.