I did contact support and what I got was not very helpful at all, it's really frustrating.
Paraphrasing.
Me: My success score is low at 'effective communication' and 'client satisfaction'. Despite I got returning customers with 5 star public review. I went to study the system and each metric and I don't think I know of any mistakes I made in those areas.
CS: (explains meanings of each metric...) also you shouldn't request 5 star reviews from clients (violation of TOS blah blah blah), instead change to requesting honest feedback in private for your own improvement. And yes, the customers rated my work in private, you're not supposed to know which ones rated you positively or negatively in each metric.
"Keep in mind that each Gig’s history is examined to determine an individual score, relative to other freelancers in the same category and price benchmark. Consequently, your Gig's rating is influenced by the performance of other Gigs and sellers."
Me: I've never requested a positive review, and WHAT? I can get negative ratings without knowing which areas??? [To myself: then how the hill I'm going to find a way to improve?]. Ok so how much do I have to improve those stats?
CS: The success score considers your performance in the last few months but we are unable to share this information as it is internal.
Me: [To myself: Oh, I can't know how long those ratings last and they are also dependent on other sellers' performance? Wow Fiverr wow this is a huge thorn for self-improvement]