Use the Academy to learn about your profile and your proposals. You are likely receiving less than stellar private feedback, and need to work on why. If you work hard, learn what wasn't great, you can find jobs and get your JSS improved.
It would not serve the clients if the freelancers could hide poor JSS.
Like jeanne said, the client can leave a private review of your performance, you can't see that review but it will be reflected in your score, and like she said try to understand what you can do better if is the quality of the work the speed, or the politness, what ever it is is making your client not too happy, even if they leave you 5 start they can still not raccomand you for future projects and that ruins your score
Jeffry's correct! That's one reason for your JSS to decrease. In addition to that, I want to clarify that your JSS can change because: