Hello, I don't have Windows 7, but I do have a PC with Vista, so should be similar. I have not had any issues flashing from the OTG port(USB-C). The thing that seems critical for me, is how long the Power button is held after pressing and releasing the Reset button. I find when I actually count the seconds, Vim is recognized every time.
Sometimes it is a good idea to turn off the Windows PC's USB port power saving mode.
On a different device, uninstalling the World Cup driver, restart PC and re-install driver, helps.
Hope you get it going.