Upwork deliberately obscures the specifics of how your JSS is calculated to make it harder to game/abuse the system, but they do outline the factors that influence it here.
This section in particular might be of interest to you:
More than 90% of freelancers have a score after 5 projects. Since not all projects and clients are equal, the length of time will vary. Nearly all freelancers have scores after completing eight projects. The projects need to be with at least three different clients and take place within a 24-month period.
Many of your recent jobs were low-cost fixed rate jobs, and "not all projects ... are equal". Keep doing good work and getting good feedback, and the score will come eventually.