I had the same issue, mac not booting after RAM upgrade. I put the RAM sticks in the empty slots, but wouldn’t boot up. I took each of the new RAM out one by one and swapping them in the available slots, and figured that my second slot (from the bottom) is not working. I then decided to put the existing RAM in those sockets, to see if it does the same….nope, it booted. So then i added the new RAMs (in the sockets where the original sticks were) and it finally booted up and recognized all of them. Don’t know why it did this, but in the initial configuration it would boot with only one added stick in the top socket.