Sourdough

I'd definitely appreciate any advice people can offer.

I've just baked a boule of the basic sourdough bread and it came out pretty tasty. There's one downside: after I let it cool for a few hours on a wire rack, the bread's solid crusty shell softened along with the rest of the boule itself. What looked like a hard boule with the crisp, outer shell of a good baguette became soft and almost cakelike after cooling. It tastes fine, but it's unlike the crispy boules I've seen in good bakeries.