What is the input frequency? Maybe your inductor value is too big.
Are you driving it with enough power. Probably need > +23 dBm.
-4 volts may be too big. Might need to be smaller, maybe near zero.
The inductor serves 2 purposes. First, it is a simple lowpass filter, so that the higher frequency spike energy goes out the output, and not the input. Secondly, when the srd abruptly shuts off, it is the inductor itself that forms the voltage spike (due to the collapsing magnetic field).
One thing you can do is to remove the transmission line pulse forming network temporarily and work only on the SRD portion of the circuit. When you have a big pulse coming out of the SRD portion, then add back the transmission line circuit to make it narrower.