Hi Sylvia! 

When bread rises well at first but sinks down just before or during baking it is generally caused by over rising the dough. When the dough over rises, the structure of the bread is compromised. The gluten strands become weak and cannot hold themselves up anymore, so the bread collapses or sinks. It's very similar to a balloon that's been overinflated. 

One of the best ways to tell if your dough is proofed is the poke test. Press one finger gently into the dough, about 1/4" deep. If the dough reacts slowly, leaving a small indent where your finger was, it's ready. If it bounces back right away then it needs more time. If it leaves a big hole, or if the dough begins to collapse, it's risen too much. 

Our blog shows some great photos of what this can look like, including loaves that have gone a bit too far. 

http://www.kingarthurflour.com/blog/2014/06/20/the-bread-also-rises/

I hope you find this helpful and happy baking!

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