You start GTM preview mode, but Tag Assistant won't connect or doesn't see your site. Before treating it as a container bug, know that the cause usually sits on the browser side.
Common causes
- Blocked third-party cookies. GTM preview relies on cookies that some browsers (Safari, strict Chrome settings) block. This is the most common reason it won't connect.
- Ad-blocker / extensions. They can block the scripts preview depends on.
- Consent. If the banner blocks measurement, tags won't fire in preview — grant consent.
- Wrong window / domain. Preview must run on the same domain you're debugging.
What to check
- 1. Allow third-party cookies (or test in Chrome with less strict settings).
- 2. Disable ad-blockers and extensions for the domain under test.
- 3. Grant consent in the banner so tags can fire.
- 4. Confirm you're opening preview on the correct domain.
Once connected
After preview connects, verify the event flow (site → server-side container → GA4 DebugView). The overall verification routine is in how to verify server-side tracking works.