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.
Building a reliable PC involves more than just assembling the parts. You need to be able to configure all of the BIOS settings appropriately. This can be quite involved and frustrating as it can require a lot of trial and error with stress testing. It is however often the only means to get a 100% reliable PC.