Plotting Harmonic Product Spectrum

I used Harmonic Product Spectrum to find the fundamental note present when a number of harmonics are present. This is the code I've implemented;

    [song,FS] = wavread('C major.wav');
    %sound(song,FS);

    P = 20000;
    N=length(song);                     % length of song
    t=0:1/FS:(N-1)/FS;                  % define time period

    song = sum(song,2);
    song=abs(song);

    %----------------------Finding the envelope of the signal-----------------%
    % Gaussian Filter
    w = linspace( -1, 1, P);                      % create a vector of P values between -1 and 1 inclusive
    sigma = 0.335;                                % standard deviation used in Gaussian formula
    myFilter = -w .* exp( -(w.^2)/(2*sigma.^2));  % compute first derivative, but leave constants out
    myFilter = myFilter / sum( abs( myFilter ) ); % normalize

    % fft convolution
    myFilter = myFilter(:);                         % create a column vector
    song(length(song)+length(myFilter)-1) = 0;      %zero pad song
    myFilter(length(song)) = 0;                     %zero pad myFilter

    edges =ifft(fft(song).*fft(myFilter));
    tedges=edges(P:N+P-1);                      % shift by P/2 so peaks line up w/ edges
    tedges=tedges/max(abs(tedges));                 % normalize

    %---------------------------Onset Detection-------------------------------%
    % Finding peaks
    maxtab = [];
    mintab = [];
    x = (1:length(tedges));
    min1 = Inf;
    max1 = -Inf;
    min_pos = NaN; 
    max_pos = NaN;

    lookformax = 1;
    for i=1:length(tedges)

        peak = tedges(i:i);
      if peak > max1, 
          max1 = peak;
          max_pos = x(i); 
      end
      if peak < min1, 
          min1 = peak;
          min_pos = x(i); 
      end

      if lookformax
        if peak < max1-0.07
          maxtab = [maxtab ; max_pos max1];
          min1 = peak; 
          min_pos = x(i);
          lookformax = 0;
        end  
      else
        if peak > min1+0.08
          mintab = [mintab ; min_pos min1];
          max1 = peak; 
          max_pos = x(i);
          lookformax = 1;
        end
      end
    end

    max_col = maxtab(:,1);
    peaks_det = max_col/FS;
    No_of_peaks = length(peaks_det);

    [song,FS] = wavread('C major.wav');
    song = sum(song,2);

    %---------------------------Performing STFT--------------------------------%
    h = 1;
    %for i = 2:No_of_peaks

        song_seg = song(max_col(7-1):max_col(7)-1);
        L = length(song_seg); 
        NFFT = 2^nextpow2(L); % Next power of 2 from length of y
        seg_fft = fft(song_seg,NFFT);%/L;

        f = FS/2*linspace(0,1,NFFT/2+1);
        seg_fft_2 = 2*abs(seg_fft(1:NFFT/2+1));
        L5 = length(song_seg);

        figure(6)
        plot(f,seg_fft_2)
        %plot(1:L/2,seg_fft(1:L/2))
        title('Frequency spectrum of signal (seg_fft)')
        xlabel('Frequency (Hz)')
        xlim([0 2500])
        ylabel('|Y(f)|')
        ylim([0 500])

    %----------------Performing Harmonic Product Spectrum---------------------%

       % In harmonic prodcut spectrum, you downsample the fft data several times and multiply all those with the original fft data to get the maximum peak. 
        %HPS
        seg_fft = seg_fft(1 : size(seg_fft,1)/2 ); 
        seg_fft = abs(seg_fft);
        a = length(seg_fft);

        seg_fft2 = ones(size(seg_fft));
        seg_fft3 = ones(size(seg_fft));
        seg_fft4 = ones(size(seg_fft));
        seg_fft5 = ones(size(seg_fft));

        for i = 1:((length(seg_fft)-1)/2)
            seg_fft2(i,1) = seg_fft(2*i,1);%(seg_fft(2*i,1) + seg_fft((2*i)+1,1))/2;
        end

         %b= size(seg_fft2)

        L1 = length(seg_fft2); 
        NFFT1 = 2^nextpow2(L1); % Next power of 2 from length of y

        f1 = FS/2*linspace(0,1,NFFT1/2+1);
        seg_fft12 = 2*abs(seg_fft2(1:NFFT1/2+1));

        figure(7);
        plot(f1,seg_fft12)
        title('Frequency spectrum of signal (seg_fft2)')
        xlabel('Frequency (Hz)')
        xlim([0 2500])
        ylabel('|Y(f)|')
        ylim([0 500])

This is the plot for Figure 6

So in the real scenario once I perform HPS (downsample by 2) the peak at 440.1 should shift down to 220 while the peak at 881 should shift down to around 440. But when I plot the graph that is not what I get. Insetad this is the graph I get,

Why don't I get the correct graph???? I don't seem to understand what Im doing wrong here... Can someone please have a look and let me know.. Thank you.....