If you would like to to keep your processes after server restarts you need to use persistence and use dedicated HumanTask handler that will trigger process isntance to continue with the flow. So to do that you need:
- build/load your sessions using JPAKnowledgeService
- use CommandBasedWSHumanTaskHandler
- after restart, make sure you have loaded back session before completing the task
Once you have this in your application your process should work as expected.
Take a look at documentation about jBPM persistence.
HTH