geometry package in combination with
align environment is not working in all pages. When many lines are within the
align environment they get automatically shifted onto the next page and the bottom of the previous is left blank.
I think you are using align environment from
amsmath and that there are many lines of equations inside align. If they come at the bottom of the page, they won't break across the page. You have to add
\allowdisplaybreaks in the preamble (say just before
\allowdisplaybreaks %% Add this