I’m facing the same problem!!!
I’m fairly sure it is code related - even though the programs compile with no errors.
I’ve resorted to having to alternate the D7 LED between HIGH and LOW each time i flash a new version of the program – just to make sure its not one of those “fake” flashes which doesn’t actually accept the new firmware. 
I’m actually just in the middle of trying to provide an extremely simple program that does this consistently – so the Spark peoples can debug it. Brb.
cya
R