Find lines using Hough transform

I am trying to find the lines using Hough transform in the image (<http://i.stack.imgur.com/4nBqS.png)>. After running it does not detect all the lines, why? and what do I have to change to fix it?

The script:

I = rgb2gray(imread('http://i.stack.imgur.com/4nBqS.png'));
BW = edge(I,'canny');
imshow(BW)

[H,theta,rho] = hough(BW);

P = houghpeaks(H,5,'threshold',ceil(0.3*max(H(:))));

x = theta(P(:,2));
y = rho(P(:,1));

lines = houghlines(BW,theta,rho,P,'FillGap',5,'MinLength',7);

figure, imshow(I), hold on

max_len = 0;
for k = 1:length(lines)
   xy = [lines(k).point1; lines(k).point2];
   plot(xy(:,1),xy(:,2),'LineWidth',2,'Color','green');

     % Plot beginnings and ends of lines
     plot(xy(1,1),xy(1,2),'x','LineWidth',2,'Color','yellow');
     plot(xy(2,1),xy(2,2),'x','LineWidth',2,'Color','red');

end