I too have seen this behavior, but only when tracing processes that execute for long periods of time (e/g/ Windows Services). The configuration specifies a daily rotation, with max parts of 14. The log files will simply stop being created, and no trace data when you need it.
Note that restarting the Windows Service will immediately create the latest trace file with a datetime stamp of when the Service was restarted, and continue on for 14 days, then stops creating new files after that.
Here's the "Connection" parameter that I use in my app.config:
"file(filename="AppTrace.sil", rotate=daily, maxparts=2, append=true)"
Any chance of a resolution?