Dough that hasn’t been proofed properly will result in dense, hard bread. Both under-proving and over-proving dough can cause milk bread to turn out dense. Under-proofed bread has not risen enough, which means the yeast hasn’t produced enough carbon dioxide to create a fluffy texture. After the first rise, the dough should double in size, and it should double in size once again during the second rise. Over-proofed dough (i.e. dough that has been allowed to rise too much) means the yeast has over produced, and the bread will deflate in the oven, resulting in a loaf that loses its fluffy texture.