Universal Analytics Is Off. Here's What Broke and How to Fix It.

Cause one: the numbers changed and nothing is wrong

GA4 counts differently. Session definitions differ, the default attribution model isn't last-click, and engaged sessions are not the same thing as sessions. A drop of a meaningful percentage against your old UA numbers is expected and doesn't indicate a fault. Before investigating anything, check whether you're comparing metrics that were ever equivalent. Usually you aren't.

Cause two: the tag isn't everywhere

The most common genuine fault. The tag went on the main site during the migration and missed a subdomain, a separately-hosted pricing page, the help centre, or a landing page builder that renders outside your normal template. The symptom is a suspicious spike in direct traffic and self-referrals, because users cross an untracked boundary and start a fresh session. Check every hostname in your data and look for anything that shouldn't be a referrer.

• Audit every subdomain and any separately hosted page
• Check the referral exclusion list includes your own domains and payment provider
• Confirm conversion events fire on the actual thank-you state, not a button click
• Look for self-referrals — they almost always point at a tracking gap

Cause three: consent is working now

Many teams tightened their consent implementation at the same time as migrating. If you were previously collecting data before consent and now correctly aren't, you'll see a genuine reduction in measured traffic. This is a compliance improvement, not a data problem, and it needs to be explained to leadership as such — otherwise it gets logged as a performance decline and someone starts optimising against a phantom.

Rebuild the report you actually use

Don't try to recreate the whole UA reporting suite. Identify the handful of views that inform decisions — usually acquisition by channel, conversions by channel, and top landing pages — and build those properly as explorations. Everything else was probably never looked at. A migration is a rare chance to reduce a reporting surface rather than faithfully reproduce a decade of accumulated clutter.