I just checked the book that came with my Toastmaster Bread Box bread maker and it said that if the top of the loaf is sunken, it could be caused by too much moisture. The suggestion was to reduce the liquids by 1 or 2 tablespoons the next time you make the bread.
Also, it was suggested that if fruits or vegetables are added to the bread, that you should drain them well and pat them dry before adding them. Hope this helps.
Bob
February 2009