help me debug - how to prevent code from collating with for loop?

Hi, I have this code that goes like this and I do not understand why the problem matrix is collated i.e. it gives a size of 22010 x 1, when the second line of code is only meant to extract a matrix from a cell array (answer should be 270x1, or 280x1 depending on A). Is there a way I can ensure that the second line of code does not collate but rather extracts the specific matrix from cell array locationsacc? Thank you!

Additional info:

locationsacc - 8x1 cell

A - 66x1 double

sessions_fef - 131x1 double

for j = A
    problem = cell2mat(locationsacc(sessions_fef(j,1))); %%why is it 22010!!!
    firing1 (:,j) = accumarray (problem, dataset_fef_sac_all(j,:)); %%collated firing rates for 8 locations
end 

[Stephane]

You can break down the problematic line to several lines of code, with extra variables. One of the three calls "cell2mat(locationsacc(sessions_fef(" is faulty.

[Cside]

thanks! i managed to debug it :)