In the callback function corresponding to the button that opens the Excel file, please verify that you have called "fclose" after "fopen". The callback function after correcting it should look something like this:
fid = fopen(FileName,'r');
% perform other actions here using the open file
fclose(fid);
The first time you press the button, you are able to save it since that is the first open instance of your Excel file.
During the second time you open the file through your App's button, there is already an open instance of the same file inside the MATLAB process which prevents you from making changes to the file. This is despite you closing the Excel file window from the first instance. Calling "fclose" on the file identifier inside your callback function should solve the issue.
Please note that it is generally a recommended practice to call "fclose" on a file identifier opened using "fopen" after reading/writing to it in MATLAB.
