I've been with Alamy over nine years now and never had a fail in all that time (touch wood etc.) but I'm still on just three Q stars, like many others.   From what I see of those people on five stars I've come to the conclusion that the main difference is the greater volume and frequency of submissions and/or having Live News submission privilege. I seem to remember the five star QC ranking became a thing about the same time many ordinary contributors, like me, had Live News privileges