You either aren't adding enough yeast (or your yeast is no longer any good - do you store your yeast in the fridge?)
or...
You are baking the bread too long - so the bread really does rise, but then as it bakes for beyond what it needs the air inside escapes, causing the bread to fall flat.
We have a breadmaker machine at our house, which makes life much easier in that regard.. I think I've only done bread in the actual oven once, and that was in college. [ MFS's advice column | Ask MFS A Question ]
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