You do not seem have the input matching network included in your simulation, unless it is already contained in the S-parameters of the device. From experience I found that, even though there is some reverse isolation between input and output due to the transistor, a variation in the input matching can have a good deal of influence on the amplifier S22, i.e., the impedance looking into the amplifier output.
And clearly, if you want to control gain and flatness as well, you will definitely need the input matching network in your simulation.