I then burn them to a 16GB flash disk using win-32 disk imager.
MicroSD card or USB stick? USB sticks won’t work.
Can you confirm on your hardware the images that are currently hosted are functional?
The images are all CI’ed meaning that they’re tested before being uploaded to the server.
Please see the general troubleshooting page. Your issue is either flash corruption or power if more than one board is experiencing the same issue. The green LED should turn on if the bootloader was flashed correctly.