Dense crumb only in the middle of the loaf
I've been baking a few round loaves recently, following the formula in Tartine Bread for the country loaf. They turn out pretty good, but one oddity I've noticed is that the crumb is very open for the first few slices, but gets progressively more dense as I get closer to the centre (the widest part of the loaf). It's cooked properly, but is comprised of smaller, more regular air pockets, almost like a standard loaf of white bread.
So I'm wondering if anyone else has experienced similar issues. The pictures in the book suggest that the open crumb should persist throughout the loaf.
One thought I had is that the heat is not penetrating the centre of these large loaves before the outer portions have solidified, preventing proper expansion. Is that possible? And if so, what can i do about it? I'm cooking in a Dutch oven and at the suggested temperatures, which seem to produce the correct crust colouring.
Maybe it's something else entirely. Any suggestions welcome :)