check all your connectors, check if all pins are correctly soldered on both (the red and the blue) board
check your motor connections, check ESC power connections
i don't think that one of your main boards is damaged
If you have those bullet connectors between ESCs and motors, make sure to check they all have good contact.
In my case I found several of them were rotating freely also when mated, causing bad electrical contact. I soldered them up solid.
Swapped in a new APM today and the quad is back in the air. :^)
Looks like my other APM was indeed damaged or had some defect so it is off to diydrones for repair (kudos to them for customer support!)