Did you happen to unseat cpu from socket during mobo transfer? a bent socket pin can render a non responding DIMM slot sometimes.  
At first the computer originally had 2x2GB sticks and was running okay. But then I decided to change the case just to fit a bigger GPU (still using the thermal sensors, front panel usb stuff, power button). I installed 3x4gb sticks and the PC didn't boot. I would test a few sticks at a time, no boot. Codes I usually got were 1 3 and 2 4. Only slot 3 works, it just only boots/posts like this. Moving into slot 1 2 4, no work. Using 2 sticks, no work. I have spare CMOs batteries, tried that, didn't do the trick. And I'm sure the module themselves are fine, they work well. Oh and my keyboard and mouse doesn't work when it first turns on, only when Windows loads. So i can't press that F1 F2 bs when the system detects change. Now I'm not sure if it's some setting configuration or something I don't know. Or it's just possible I somehow manage to break the motherboard. Also the original 2x2gb ram doesn't work anymore.
Optiplex 790 Other RAM Slots will not work except slot 3