prior to NTA 4.1 - there were known issues on previous version that NTA flows were not being deleted during database maintenance. it was happening every 12mn. However that was resolved and on NTA 4.1 and above. However, that being said, even if that was fixed on the said version, the files may still retain on that path location. Solution is to manually delete those files. Files that you see on the data way pas the retention period you have set.
You just have to stop the netflow service on the Flow storage database server >> under Services.MSC >> then go to the path location and manually delete those files. Then go back and launch NTA flow configurator and start it there.
This should solve your issue.