I have written thousand times. S-parameters are small signal parameters and they cannot be used nonlinear circuits. Therefore S21 is almost zero due to Open Circuited Diodes.There is just small junction capacitance but the Diodes are Open Circuit.Simulator drive the circuit with very small negligible voltages and currencts.( no sense at all )
If you would check the Power Gain, use Large Signal S-Parameters Simulation with appropriate driving level as same as HB level.
HB will also give the Power Gain, why don't you use this feature ?