With only two of your clients entering reviews, it is surprising that you have a JSS at all. The client who gave you less than 5 stars probably gave you a less than positive Net Promoter Score (the private feedback). So, that will hurt. Plus, your percentage of non-reviews is probably high since you have so few clients total. If your JSS falls below 50% it will no longer be visible to clients.
You should work toward allowing clients to close the contracts. When doing so, they are required to leave a review.