Not enough indexing
We have installed wpForo as a question / answer modification but we have so few pages indexed on Google as seen on screenshot.
Over 100 questions, Google only indexed 13 of them. Only question-link/newest or link/older kind of links are indexed. We don't know why this happens.
Well Google is not obliged to index anything.
And definitely this has nothing to do with wpForo plugin.
It takes time, VOTES (backlinks) and work. For everything related to data on the web.
Make sure you inserted your site map in Google Webmaster tools: https://www.google.com/webmasters/
Your sitemap URL is located in Dashboard > Forums > Settings > General Tab. Please see the screenshot: