Good Morning Wormy,
I have a document listed at the link below that should help. There are a few ways these locks are not released,
1. QTP Crashes and closes
2. QTP's connection to the QC server is interrupted (VPN timeout, Server connection time out, etc)
3. Closing QTP through the task manager
Basically if QTP isn't closed correctly this can happen.
[Manual Release of QC locked QTP Tests]