Got it! I think with many it has to do with the maturity of the plant. I do not get many plants to bloom in clusters as you suggest, but we also have such a short growing season here in Wisconsin.
Bette Davis Eyes does well here, this is probably a 8-10 year old clump that I divided into 3 clumps and it has filled in
Gaudy Gaudy bloomed pretty good last year with many blooms open at one time
Untangle My Emotions puts on a good display
Fuchsia Fashion is a fun one!
I adore So Lovey, nice and tall pale yellow(just to left of blue birdbath), heavy bloomer like this for almost a month
Edited to add: I see you're in a much warmer zone, don't know how these would do in your climate