Question RAM only works in slots 1-2

[Morlow]

I just bought 32 BG (16x2) of RAM to upgrade my PC but it appears that RAM only works in slots 1 and 2. I obviously want to keep the 16 GB (8x2) that were already in the machine but I can't get any combination to work except 1-2. My RAM is compatible with my MOBO and they are exactly the same thing, just 32 GB vs 16. I have been trying to find someone with the same situation with Google but can't find anything. Do you think my MOBO is toast or should I try to reseat my CPU? Not sure where to start.

I should mention that the RAM has been in the computer in the 1-2 slots since I built it (I didn't realize it would be slower that way).

 

[rgd1101]

and the motherboard model?
exact model of these ram?

 

[Morlow]

MOBO: MSI Z270-A Pro

RAM: Corsair Vengeance LPX DDR4 2x16GB 2400MHz ...aaand the old memory isn't exactly the same:
Corsair Vengeance LPX DDR4 2x8GB 3000MHz

I thought they were both 2400MHz. Both are supported but maybe they won't work together. I know my MOBO will slow it down to 2400 so I'm not sure why I bought 3000 originally.

 

[rgd1101]

and you try just the old ram on different slot?

 

[Morlow]

Yeah, it only works in 1&2 like the new RAM.

 

[rgd1101]

sound like the motherboard

 

[Morlow]

Yeah, any time I put any stick in slots 3 or 4 I get the debug lights.