how to increase gain of 8 element microstrip array on hfss ?

hi evry1,
i'm supposed to design 8 element microstrip array antenna wit gain of 15dbi at 5Ghz and substrate of dielectric=2.5. Im gettin decent VSWR of 1.5 at 5ghz, though i'm able to get only 13.5 dbi gain. I'm using proximity coupling wit corporate(parallel) feed network. So pls let me know how to increase gain to around 15dbi. I've to submit in nxt week, so do reply soon guyz plzzzz.

Im not to sure about HFSS but in MWS you can optimise an antenna for gain.

Its probably necessary to do that and it may take a quite a time. Maybe an overnight run.

Thanks.. but i hav only HFSS!