OpenWRT question - FW upgrade rejects on some devices

Hi, I got several N600 routers I got from different sources.
I try to upgrade them to OpenWRT (using 15sec reset to get to the FW upgrade page)…
Some routers agree to upgrade to OpenWRT, and some refuse.
Any clue why?

It would be best to consult with software provider for OpenWRT for additional information since there are no tutorials or documentation files available within the WD Community.