-You load you amplifier with 50 Ohm oscilloscope impedance and the amplifier cannot drive this impedance.Because optimum load impedance of this PA is probably very different than this
-Your PA has no impedance input/output matching circuit and there are so much matching losses.A PA is not designed and implemented in this way..
-Variable Pots are never used in RF circuits, never...
-PAs are very crcitical circuits and they should be designed and implemented very carefully.