I have made yeasted bread for over thirty years, and became interested in making sourdough about a year ago. I follow the Tartine method, and until a few weeks ago was consistently turning out loaves that looked and tasted how I thought they should be. Strong gluten windows when folding, resulting in lots of air pockets, some quite large, and a loaf that expanded rapidly once in the oven. Something has happened, though, and lately my dough just slumps once I tip it from the basket onto the stone for baking. As a result, the loaves are quite flat (see photo) and have a very uniform distribution of air pockets. The leaven that I used for the loaf in the photo looked as it should, and floated when I added it to the water, but the starter itself seems to be active for shorter periods after feeding and has a stronger (but not unpleasant) odour. Since the dough has started slumping I have not been able to make a loaf that looks like my earlier successes, no matter what I try changing. I'm beginning to think that there is something wrong with my starter. Any suggestions?