Pagination Problem

I have put this in the loop in order to display post excerpts in different formats, it work but when i added pagination it simply displays the same posts on subsequent pages as it does on the home page.

<?php if (in_category('two_columns_right') ):  // we're in the two columns right category ?>

<?php elseif (in_category('two_columns_left') ):  // we're in the two columns left category ?>

<?php elseif (in_category('two_columns_text') ):  // we're in the two columns text category, two_columns_text ?>

<?php elseif (in_category('single_column') ):  // we're in the single column category, two_columns_text ?>

<?php endif; ?>