I have some issue with the upload of sketch on a sparkfun pro micro board with the processor ATMega32U4 (3.3V, 8MHz).
It seems that my board is/was bricked. Actually, i'm able to upload a sketch but it needs ugly bootloader mode. The working sequence I am using now is :
State = COM6 on device manager & COM6 selected on Tool>Port
Double hit reset button : Bootloader mode activation
State = COM5 on device manager & COM6 selected on Tool>Port
Double hit reset button : Keep bootloader mode activation
Wait for Found Upload port COM5
Keep double hitting on reset button while the sketch is uploading (sometimes I double hit 3 or 4 times before the sketch is uploaded)
If i have the good timing (if I don't double hit when the sketch uploads) the upload works. Otherwise I have a "can't open device COM5" because of the double hits.
By the way, sometimes when I reactivate bootloader mode, the board doesn't start correctly and is no longer known by my PC which cause the failure of my sequence.