You have to choose the bounty that has a good and reliable manager. If you are feeling depressed that your efforts were wasted that's the other side of bounties.
If they aren't able to reach their soft/hard cap they will never pay your efforts so if you want to get paid with the bounty you are joining. Just choose the bounty that has an experienced bounty manager and assures the participants to get paid.
Please tell me which are the best bounty campaign managers which i can trust
This list
Overview of Bitcointalk Signature Anti-Spam Campaign ManagersThese managers are reliable in the forum so if you see them have a bounty, you can trust their management. There are some separated list but if I were you, I'll rely on those managers.