I've noticed that for quite some time now, Google has indexed only half of my sitemap. The site is on the larger side, and has been around for a long time, so I don't believe it's a time issue. My other XenForo 1.4 sites are nearly 100% indexed, and I'm worried about my SEO.
Could I have something mis-configured? I don't have a robots.txt
Google will usually tell you if pages in the sitemap have been blocked. What Google decides to index is mostly just going to be down to their own internal decisions so we couldn't explicitly tell you why it has or hasn't indexed specific pages.