ADS design : Amplifier designing dbt

hi
i am trying to trouble shoot an amplifier design.I have a layout with 50 ohm line and two dc blocking capacitance in series and two shunt quarter wave lines (converting short ckt to RF open ckt ) for biasing amplifier. Amplifier i/p and o/p impedance are 50 ohm.once i simulated it in agilent ADS my o/p gain variation is not uniform over my freq range 3.1 to 3.5 GHz(showing a fluctuation of even 5 db,max gain of ampr is only 11.5 db).Wat shall be the error and hw can i eliminate it. How to trouble shoot it



thanks in advance