Headers & Footers overwrite body text

I am trying to compile to PDF format for CreateSpace. I have the book title and author name in a header and the page number in a footer. When it is compiled, the headers and footers overwrite the text in the body on my book. I have margins of 0.75 inches on the top and the bottom, so there is plenty of room for the headers and footers above and below the body text. Any ideas what I need to change to fix this? I am doing all this on a Mac, not Windows.

I figured out the solution myself. It turns out when I defined a custom page size, I set the printable area so that the upper and lower margins were the same as I set in the page setup. One I changed these to 0.25 instead of 0.75, everything worked correctly.