Experimenting with Andrew Whitely's really simple sourdough formula I continually get a split at the shoulder of the loaf. I thought I might not be proving long enough or too long, so I've done two loaves with significantly different proving times. The picture shows 2 loaves: the one on the left proved for 16 hours and the one on the right proved for 8. The difference in proving time doesn't seem to done anything. Next step, I suppose, is to try a stronger flour.

Update:

Here's the same loaf without the seed crust. It's been slashed but still split at the shoulder.