There are a few reasons why ginger cookies might go flat while baking. One reason could be that the dough was not chilled before baking, which caused the cookies to spread too much. Another reason could be that too much flour was used, which made the cookies dense and heavy. Finally, over-baking can also cause ginger cookies to become flat and dry. To avoid these issues, make sure to chill the dough for at least an hour before baking, use the correct amount of flour, and do not over-bake the cookies.