Unless these AM3+ mobos are fairly recent production they may not have the correct BIOS to recognise the FX processors, which would result in no response on boot. If the other CPU you tried that works is in fact an FX CPU, then yes it's possible for this FX CPU to be bad, but they are all tested before packaging so it would need to be in-transit damage or electro-static discharge, etc.
It's amazing that you can't e-mail AMD for warranty replacement as in the past... I don't know what the deal is but it's unacceptable, IMO.
Technical ignorance is NOT a destiny it is a choice. Do your homework so that you can make technically informed decisions and not be duped by advertising hype or mis-information stated as fact when its not.