Sourdough

Recently, I tried baking the 70% Hydration Boule as outlined in the video and on the website. The bread was a failure and looked like this:

My first guess as to the cause of the failure, is that the starter may not have been quite active, or mature, enough to lift the dough. The starter was only about 10 days old and was not yet on a regular feeding schedule. However, it passed the float test right before baking. When I looked back at the bowl I floated the starter in 20 minutes later, it had eventually sunk.