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.
I read that it could take 30 seconds or more for the computer to initialize ram if some was added or the sticks rearranged. Did you wait at least 30 seconds after powering on with the new ram added?
I can only tell you what worked for me on my late 2009 iMac. After much frustration I pulled out the plastic tab all the way and at the same time inserted the new RAM with considerable force to make sure it was seated. Trust me you'll know when it's seated properly because it won't move at all.
If you have inserted a RAM stick in slot 4 and 1/2/3 it does not boot, and that stick works in slot 1/2/3 and you have inserted it properly it is perfectly possible.
You can also try resetting the SMC by disconnecting power and waiting 15 seconds and then plug it back in, wait another 5 seconds and turn it on.
And NVRAM / PRAM by turning off the mac and holding CMD+ALT/OPT+R+P and turn it on and hold until you see the progress bar. Please check Startup Disk, Display, and Date & Time settings as they may have been effected by the reset."
