Hi Jane. Are you using one plugin on one site and the other plugin on the other site? Or the same plugin on both sites? Just out of curiosity, what happens if you switch to a default WP theme like Twenty Fifteen? Both plugins have a number of different options so might be hard to tell what’s going on without actually seeing the plugin configurations. Just had another thought: do the pages that are missing have page titles? Looking at a support topic on the WP Sitemap Page forum that is also asking about missing pages.