GTM is not able to send the event to GA if "you use ad blocker" or "you open another page before sending the event to GA".
Also, There is processing time.
Please check GA4 in 24+ hours (a day or 2 days).
https://support.google.com/analytics/answer/11198161?hl=en
https://support.google.com/analytics/answer/7084038?hl=en#zippy=%2Cこの記事の内容%2Cin-this-article