Don't you just hate it when something works the first time, and the first time only?
My first sourdough loaf turned out lovely. But every loaf since has burst on the bottom. I think maybe underproofing, largely due to a very cold home (I don't have central heating) and a probably inaccurate oven.
I take my starter out, feed it, and let it warm up to room temp. I mix in approx 500g flour with 200g starter and water to make a nice dough. I let it double, shape, and let double again. And every time (since the 1st) I've had the bottom and sides burst.
Please take a look at the photos and give your input!