@LittleMissSunshine Yep, that sounds exactly like what happened with my first few tests! I'm using a 9oz straight sided jar. I started with a CD 10 and 12 just like you. The CD 10 seemed like a great size flame on the first burn, but didn't get to a full MP in 4 hours. The next day when I relit it, the flame was flickering a lot, and just streaming soot. I had poured these test candles without wicks so I could just poke a hole in the solid wax and swap out wicks as needed. I definitely recommend trying that if you haven't.. saves wax/jars/wicks/sanity to a degree lol. I think it was @Flicker that mentioned she does this, and it's been a huge help to me!
So anyway, I switched to a premier 775 for my next burn and while I got zero soot, my melt pool was no closer to being full at 3-4 hours, and tunneled again. So now at this point I've tried CD 8, 10, 12, and Premier 735, 745, 750, 755, 775, 780, 785. None of which have yet given me the non-tunneling melt pool my heart desires. So I'm not totally sure where I'll go next from here yet.