On the left is a mammogram of a fatty breast. Since fat is radiolucent it produces a dark background which makes the small, white stellate tumour in the center of the breast relatively easy to see. The mammogram on the right shows a very dense breast. It too has a cancer in roughly the same location but I challenge you to find it.
To complicate the situation further, it appears that even among women of a given age, mammography is less sensitive in BRCA1 mutation carriers than in women with sporadic breast cancers.