It's hard to say what it is without more information. The most common issues:
- You need to activate SWV in debug configuration
- You need to set clock correctly
- You need to configure and activate the SWV trace in the debug session (red button)
- You need to make sure the variables you're tracing can be traces by SWV
There are some other things to think about, it might be best to read the SWV chapter of our user guide which you can find right here.
