I just implemented Universal Analytics and GTM for a client. I saw normal traffic after I had just implemented GTM, but when I pushed UA live, about a week later, I saw pageviews decline sharply. 45% of visits aren't registering any pageviews at all, and are coming in with a landing page of (not set). By isolating factors, I found that 100% of these sessions with no pageviews generated events. There are, however, some sessions with events that DO register pageviews - about 25% of total events have normal landing pages. There's no differentiation between sources, mediums, mobile devices, browsers... whatever it is is affecting every source and kind of traffic, but seems to be especially connected to events.
I believe I have cross-domain tracking set up correctly, and have checked the UA and GTM implementation multiple times.
Another weird thing that I noticed is for these sessions with a landing page of (not set), when you look at the Page Title instead of the URL, it actually shows up. These sessions are still registering 0 pageviews, though. One thought is that a lot of their pages seem to have one version with a / at the end, and a version without the / at the end- in other words, it's not defaulting to one version. The site is also mostly on https, and although it doesn't appear to be a problem when you go to the site, when I run the http versions of the URLs through a status checker, it comes up with a 301 error. Not sure if this is related or not. The main connection seems to be with event tracking, so any advice there would be greatly appreciated! Thank you.