Nothing is more frustrating than getting all the way through most of the sourdough bread making process only to have your dough deflate right before you're about to put it into the oven. Trust me, I've been there more times than I can count - especially as a new baker.
If your dough is deflating prior to baking, the most common reason is over fermentation (where the dough has been left to rise for too long), but here are a few other possible causes + how to fix them:
Weak gluten structure: if your dough doesn't have enough gluten development it can lead to weak strands and eventually cause it to collapse.
How to fix: make sure you are performing at least 3 stretch and folds during the fermentation phase. Also be sure you are not over or under stretching your dough.
Temperature of the environment: If your kitchen is too warm, the dough will over ferment quickly.
How to fix: If you live in a warm climate or have a warm house, find a cooler spot to place your dough while it ferments. I sometimes find that putting an ice pack under or on top of the bowl helps with this.
Weak sourdough starter: An immature sourdough starter or one that isn't active enough might not produce enough gas for a good rise in your dough, but if it's too active, it can over ferment the dough rapidly.
How to fix: Adjust your sourdough starter feeding schedule - if it's too active, start feeding it less, if it's not active enough, increase your feedings to every day or a few times a day. You may also use less starter in your dough - but keep in mind that this will increase the fermentation time.
Hydration: Dough that is too wet (has too much water) might not hold its shape as well, causing it to deflate easily.
How to fix: Try using less water in your recipe (start with 25 grams).
Improper shaping technique: if you're working your dough too much (being too rough) during the pre-shaping and shaping steps it can weaken it.
How to fix: Be sure not to overwork your dough, move slowly and gently!
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