Error in plotting the Histogram of Multi-taper estimate

Hello, I am trying to plot the histogram of the power estimate obtained using the Multi-taper method. When 1 taper is used the histogram shows the expected exponential distribution ; on increasing the number of tapers it is expected that the distribution should show Gamma distribution (as seen when shape parameter = 2 and scaling parameter = 2.0 ). However the the plot still shows exponential distribution as seen when single taper is used. Following is the code that I have used.

{

Fs = 2000;

T1 = [];

S1 = [];

for i =1: length(timeVals)

      if (-0.3000<= timeVals(i)) && (timeVals(i)<= -0.1000 );

          T1 = [T1,timeVals(i)];
          S1 = [S1,signal(:,i)];
      end
  end

Ta =(max(T1)) - (min(T1));

f1 = 0:1/Ta :Fs;

C1=S1';

params.Fs = Fs;

params.tapers = [2 3];

params.trialave = 0;

params.pad = 1;

[S,f1] = mtspectrumc(C1, params);

S1 = 10*log10(S/max(S));

semilogy(f1,S1,'k');

grid on;

% xlim([0 50])

xlabel('Frequency [Hz]')

ylabel('Power [dB]')

title ('multi-taper spectrum')

hold on;

T2 = [];

S2 = [];

for j = 1: length(timeVals)

      if (0.2000<= timeVals(j)) && (timeVals(j)<= 0.4000 )
          T2 = [T2,timeVals(j)];
          S2 = [S2,signal(:,j)];
      end
  end

Tb =(max(T2)) - (min(T2));

f2 = 0:1/Tb :Fs;

C2 = S2';

params.Fs = Fs;

params.tapers = [2 3];

params.trialave = 0;

params.pad = 1;

[W,f2] = mtspectrumc(C2, params);

W1 = 10*log10(W/max(W));

semilogy(f2,W1,'r');

grid on;
% xlim([0 50])

xlabel('Frequency [Hz]')

ylabel('Power [dB]')

title ('multi-taper spectrum')

% for plotting the histogram

figure (3);

H =  hist(W(:),100);

plot(H);

xlim ([0 100]);

}

Thank You