Image histogram fitting using Histfit()

Hello 

First of all i am a begineer in Image processing. I have some Tomographic images and each image represents different sizes of the object placed inside the sensor. I am trying to find the change in the intensities from one image to another using histogram analysis. So far i am sucessful with plotting the histograms and i am trying to fit the histogram using histfit command but what i have noticed is that the data is not fitted properly as most of the peaks are not included in the fitting. Kindly check the attached image. Here is my code:

I = imread('10mm.tif'); %loading image

I=rgb2gray(I);% grayscale% (Size 587x587)

I_V = I(:);% changing it into a vector

histfit(I_V,587,'normal')

Kindly Please suggest me what should i do to properly fit the data. Also attached is the screenshot of all of my results.