Lockable resources plugin does not take effect

We are doing automated regression testing with jenkins and using the lockable resources plugin to run one test job at a time. But sometimes it fails to acquire the lock on the testing system and one or more test job starts before the first one finished depending on when the lockable resource plugin starts to work again. Example wrong behavior:
17:09:23 Started by timer

17:09:23 [lockable-resources] acquired lock on []

17:09:23 [EnvInject] - Loading node environment variables.
Please tell me what kind of logs do you need to debug this issue.