xlsread function does not concatenate all files into one big matrix as expected

Hello,

I am working with a few .xlsx files which I want to concatenate into one, big matrix. Attached you find examples of 4 files and below you have the piece of code I am currently playing with:

 folder='D:\Documents\...'; % my folder
filetype='*_pilot_trivia.xlsx';  
f=fullfile(folder,filetype);
d=dir(f);
all_participants = [];
for k=1:size(d,1); % using 'numel' instead of 'size' doesn't help
  all_participants = xlsread(fullfile(folder,d(k).name));
end

Basically what happens is that everything goes smoothly except the fact that only one specific file out of the entire list gets loaded (i.e. only 9_pilot_trivia.xlsx appears in all_participants in the end) and I don't know why. Could you please be so kind as to help?