You may find that the .hex file fails to be flashed to your micro:bit, resulting in the file briefly appearing on the micro:bit device, alongside a FAIL.txt, before being ejected.

Like all files, .Hex filenames need to be properly formatted. So whilst punctuation (, ; :) is often found in code, it is unexpected in filenames causing the flashing process to fail. 

To resolve this, you will need to rename the file in the project settings to remove the punctuation:

Keywords for search: upload, flash, fail, hex, fail.txt, Microbit will not add new program