To those who are stuck wondering why a window flashes and goes away without doing anything, the problem may related to the RELATIVE path in your Python script. e.g. you used ".\". Even the Python script and Excel Workbook is in the same directory, the Current Directory may still be different. If you don't want to modify your code to change it to an absolute path. Just change your current Excel directory before you run the python script by:
ChDir ActiveWorkbook.Path
I'm just giving a example here. If the flash do appear, one of the first issues to check is the Current Working Directory.