Help needed in design of microstrip yagi antenna

I am designing microstrip yagi antenna and simulating it in HFSS. But even though i add no of directors the gain does not increase substantially as it should. I have also maintained the distance between the directors and feed according to the data given in some research papers. I would like to know whether data from research papers(same data also available in Balanis book) which is generally applicable to antenna with rod shaped elements suspended in air applicable to microstrip yagi antenna on PCB substrate.

To increase the gain of Yagi antenna more important than just adding more directors, is to optimize the distance between them, especially the distance between first director and driven element. Otherwise is possible the first director to behave as a reflector, and whatever more directors you add, there will be no gain increase.