Using AMP? A known bug is probably screwing up your Google Analytics - Marketing Land | The MarTech Digest | Scoop.it
If you are using the Google-backed Accelerated Mobile Pages, there appears to be an excellent chance that those AMP pages are not being tracked correctly, if you use Google Analytics.

Christian Oliveira, a technical SEO consultant, posted a long, technical explanation of the issues AMP is giving with proper tracking in Google Analytics. Malte Ubi, Google’s technical lead for the AMP project, confirmed the problems and that they have no easy solution, in tweets today.