@Sam27 Is this issue occurring for all the users or only a specific user. For specific user means you can Log-Off then clear browser cache and log in again and try. When multiple tracer sessions are opened some times this issue will occur. 

@Sam27  Addition to what @KotaC382 mentioned, you can also try any of the following:

• If this is happening to only specific user, terminate all the requestor session of that user, clear browser cache, and start a new session.
• If this is happening to all user, restart the server, clear temp folder and marker files as the tracer file might be full (for this when tracing ensure that tracing events is less 50000)

Would be helpful if you could please describe the scenario - when and how this is happening - so we can find appropriate solution. Thank you.

Not sure if this is of same issue Cannot Launch Multiple Tracer Sessions for a requestor | Collaboration Center (pega.com)