There could be several reasons for this issue. One common mistake is using the same value for the user_id and client_id fields when sending events via the Measurement Protocol. While both identify users, they serve different functions. Google Analytics 4 uses client_id to identify unique browsers and devices, while user_id is used to identify unique users across devices. Therefore, if you use a user_id as a client_id, Google Analytics 4 may fail to recognize and track unique users properly.

Please ensure you’re using the correct format, check for any errors in your measurements protocol setup, or potential discrepancies in how your events are categorized. There could be a potential timeout or delay in processing the data sent via Measurement Protocol, thus users might not immediately appear in your GA4 reports. Another reason can be issues with the configuration settings in GA4.

Please remember that once an event data is sent, it cannot be changed or deleted. Hence if there is an error in the setup, correct it and re-send the data. If all settings are correct, I would advise you to wait for a bit and see if data starts to show up. If you still face issues, it would be best to seek assistance from Google’s technical support.