I'm working on the "Becoming Salesforce" modules in Trailhead; something I've noticed is that it seems I can only see the first topic for a few of the modules; for example: in the "Get to know Salesforce.org" module, I see the "Giving Back at Salesforce" topic with the video with Ebony, but topics after and including "Salesforce.org and Our Philanthropic Model" aren't showing up. If I click on the missing topics in the right-hand topic navigation links, nothing happens. The quiz directly follows the "Giving Back at Salesforce", and the last question in the quiz references content that I can't find anywhere. This is what ultimately led me to discover that topics were missing.
 
Here's an exhaustive list of the modules that I see this issue with (i.e. topics after the first topic don't show up in the main page):

1. "Get Started Becoming Salesforce"
2. "Learn About Our Ohana Culture"
3. "Get to know Salesforce.org"