imfindcircles not giving answer

I want to find circles in a image,i used imfindcircles ,and used example in it which gave good result,but if i use my own image i am not getting the result,kindly assist

rgb=imread('sample6.bmp');
%rgb=~rgb;
[centers, radii] = imfindcircles(rgb,[10 55],'ObjectPolarity','bright')
imshow(rgb)
 viscircles(centers, radii,'EdgeColor','b');

image is http://imgur.com/DY0okYS