It produced no waveform output(xx) of the state machine using d flip-flop

I was trying to stimulate the state machine with d flip-flop. There wasn’t any error while compiling. But whatever the input value I set, the University Program VWF doesn’t produced any waveform output. Please help me!!

Below is my code:

module state_machine_d(Q, Qn, C, x,y);
output Q;
output Qn;
input C;
input x;
input y;

wire w1;
wire D;
wire Cn; // Control input to the D latch.
wire Cnn; // Control input to the SR latch.
wire DQ; // Output from the D latch, input to the gated SR latch.
wire DQn; // Output from the D latch, input to the gated SR latch.

not(Cn, C);
not(Cnn, Cn);
xor(w1,x,y);
xor(D,Q,w1);
d_latch dl(DQ, DQn, Cn, D);
sr_latch_gated sr(Q, Qn, Cnn, DQ, DQn);
endmodule

module d_latch(Q, Qn, G, D);
output Q;
output Qn;
input G;
input D;

wire Dn;
wire D1;
wire Dn1;

not(Dn, D);
and(D1, G, D);
and(Dn1, G, Dn);
nor(Qn, D1, Q);
nor(Q, Dn1, Qn);
endmodule // d_latch

module sr_latch_gated(Q, Qn, G, S, R);
output Q;
output Qn;
input G;
input S;
input R;

wire S1;
wire R1;

and(S1, G, S);
and(R1, G, R);
nor(Qn, S1, Q);
nor(Q, R1, Qn);
endmodule // sr_latch_gated