Turns out this is a Google Analytics issue, NOT a Tableau issue.
starrynight104, it actually turned out to be an issue with the WordPress plug-in that was being used to create the infinite scrolling feature on the site. We were able to use the Chrome Google Debug extension to track exactly what was happening. When you clicked on an article, the site passed the custom dimensions through to GA, but, as you scrolled, because of the way the plug-in was built, the custom dimensions didn't get passed but the pageviews did.
We ended up working around this by pulling the GA data and the Wordpress data into a database and then generating our Tableau reports using that dataset. Works for what we're doing, but would have been cleaner to be able to use the Custom Dimensions directly.
Hope that helps.