There can be several reasons for missing transactions in Google Analytics. The most common reasons that I have come across are:-
- Disabled cookies on the browser
- Disabled javascript on the browser
- Customers clicking away after placing the order
- Some payment gateways do not redirect customers to the order success page
- Conflicting with other extensions
The best solution that I have found is to resend missing transactions to Google Analytics using the Measurement protocol API. There's also an extension by Scommerce mage that can sync missing transactions from the previous day. https://www.scommerce-mage.com/magento2-google-analytics-missing-transactions.html
