I'd start by checking this page: https://www.dynatrace.com/support/help/how-to-use-dynatrace/real-user-monitoring/setup-and-configura...
To see 1) if any of those are being blocked and 2) if you're seeing any sign that injection is being attempted at all. In browser developer tools you should be able to see any of the indications (headers, cookies, etc...).
This likely is the reason, but you will have to check the OneAgent logs to confirm. I think I already encountered such case with unsupported IBM JVM and WebSphere in the past.