GA4 measurement

There are several reasons why an ad might show more clicks within Ad Manager compared to the number of website visitor sessions measured by Google Analytics (GA4) or other web analytics. The most common reasons are:

Click v.s. Session definition differences
An ad server measures a click as soon as the ad is clicked, regardless of what happens afterwards. Google Analytics measures a session when the page fully loads and the GA4 tracking code executes. 

Here are some examples why differences can occur:

• A user clicks on the ad, but leaves the website on the first pageview. Besides privacy consent - see below - the other reason for not registering a session in this case is that GA4 needs more interaction from the user than just a pageview.

• A user clicks on the ad but closes the tab before the page fully loads. 

• Slow page load times can cause visitors to abandon before the GA4 script fires.

• If a user clicks an ad in a short period of time more than once, it is measured as one session by GA4 whilst Ad Manager will log each click separately.

Tracking discrepancies due to privacy regulations (GDPR)

Practically all European websites have cookie consent banners. If users do not accept tracking cookies, GA4 may not be able to log their session, but the ad server still measures the click. 
Another common issue is that tracking pixels or GA4 is loaded only on page view in combination with an existing user consent. However, in many cases it is forgotten to activate GA4 tracking when consent is given after a pageview. Besides that, please make sure that consent variables are properly passed along in GA4.

Redirect & tracking issues
If there are any issues with redirects between the ad click and the landing page, users might drop off before reaching the final destination. If you use 3rd party tags or click trackers it is valuable to check if the tracking vendor measures a similar amount of clicks as Ad Manager. Improper UTM parameters or GA4 tagging might cause sessions not to be attributed correctly. It is also possible that UTM codes are not remembered when a user visits another page on the advertiser's website. If the GA4 session requires a second pageview, the session cannot be attributed to click on the ad if UTM variables are not kept.

Other

• Ad Blockers: Some browsers and extensions block GA4 scripts, but not ad server clicks. It is recommended that GA4 runs on a subdomain of the advertisers website. And also that the advertiser website doesn’t use different domains.
• Accidental Clicks: Users may accidentally click an ad and immediately close the page before it fully loads.
• Slow Internet Connections: Particularly on mobile devices, slow connections might cause the page to time out before the GA4 tracking fires.
  A user might click on an ad on one device but open the landing page later on another device, making it difficult for GA4 to track the session as originating from the click.

Troubleshooting Tips

• Check Consent Management: Ensure GA4 fires correctly after consent. Also doublecheck the configuration in your tag manager like GTM.

• Validate UTM Parameters: Confirm proper tagging and URL redirection.

• Compare with Other Tools: Use server-side logs or another analytics tool to cross-verify data.

• Review Bot Filters: Look at the ad server’s bot filtering settings and compare them with GA’s.