TBH even I can’t make my sensor mezzanine boards to work after flashing the bootloader. I have 3 boards and all reported the same issue.
But I heard that some people were able to get it work after flashing bootloader using external programmer like USBTinyISP, etc… That’s why I suggested you to try that.
I spent lot of time trying out different methods but nothing worked. My USBTiny programmer successfully programmed bootloader to the sensor mezzanine but it was not working when trying to program using CE board. So I’m suspecting an issue with the board itself.
Maybe @danielt can provide more info here.