The stretch was a bad thing, yeah. I've done a simple checkered thumb gore instead so the increases were a breeze and I didn't have to bother staying in pattern. It's doing the decreases in pattern that's the problem - since I've used sport weight instead of fingering, I have 1/3 fewer repeats of the pattern and that looked odd to me. So I shifted the thumb around so that I get the big pretty snowflakes centered on the top of the mittens. Unfortunately, that means my decreases are in the middle of the chart instead of at the sides, so I have to pay much more attention to where I am and how to make the pattern line up properly.