Header and footer margins erratic

[Rodmaru]

I am working on a book layout and having trouble controlling the depth of headers. I have set the top and bottom margins on page layout but the header usually extends down into the page 2-3 inches. When I select the lower margin and move it up closer to the top of the page, it responds by dropping down to the foot of the page, or any other position except the one i want. Very frustrating! I am a casual user and need help taming this beast.