Ian helped me fit the dress today. I had a hard time pinning it closed and then of course I couldn't pin all of the changes myself.
It turns out that my upper back is narrow. I knew that the shoulder area was narrow but I've never tried to fit something one-shouldered or strapless before. Not that I've fit this particular version of my body before, either, of course, which makes fitting a little bit more difficult.
I've made a number of changes to my pattern and I'll sew it up tomorrow. In addition to the fit changes, I also changed the side seams a bit to make them more of a right angle instead of the deep v. It'll be easier to pin myself into the dress and it'll make sure my bra stays covered. Here's hoping the next iteration of the dress fits better.