Hi Jerome, when a loaf deflates as you describe, this usually indicates that it was over-proofed (allowed to rise too long or at too warm of a temperature during the shaped rise). Often just putting the loaf in the oven a little earlier in the process, when it hasn't risen quite so much, can prove helpful here. 

However, a lot of factors can play into this result, including how successfully you shaped the loaf to begin with. During shaping you want to develop good surface tension and overall structure. A loaf that is very loosely shaped will over-proof more quickly than a well-shaped loaf. Our blogs on how to shape a boule and how to shape a bâtard can be helpful in developing these skills. Once you've managed to shape the loaf with good structure, a good test to see if your loaf is ready for baking is to gently poke the surface of the loaf with a floured finger. If the dough bounces right back, it needs more time to rise. If it leaves a print, it's probably a bit over-proofed. What you're looking for is for the indentation to fill in slowly, which is a sign that the loaf is still resilient and has some rising power as it goes into the oven. 

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