Niki

I found the solution:

<?php $args = array( 'post_type' => array( 'post', 'tribe_events' )); ?>
  <?php $custom_query = new WP_Query( $args ); ?>
  <?php while ( $custom_query->have_posts() ) : $custom_query->the_post(); ?>

I realized that this was user error, and the problem is now resolved.

Thanks for your help!