I have a problem, dual channel stopped working on my Classified motherboard, it only works on single channel. I tried two different CPU, 12900 and 12600, two different memory sets, 5200 and 5400. Nothing seems to work. Any ideas of what could it be or do i send it straight to RMA? 

Most of the time, not always, that I ran into RAM not being seen as dual channel, it was something as simple as a piece of dust in the socket.

When you switched out the CPU’s, did you verify the socket was clear of any obstructions by any chance, and that all pins looked good?

Next step, and you may have already done this, is to verify each ram module one at a time.

Are you using two dimms or 4 dimms of ram in total?

Lastly, as my most obvious checks before going ahead, have you updated your bios, and tried a CMOS reset to make sure that it isn’t some wonky bios setting that changed and caused the ram slot to not appear?

the_Scarlet_one
Most of the time, not always, that I ran into RAM not being seen as dual channel, it was something as simple as a piece of dust in the socket.

When you switched out the CPU’s, did you verify the socket was clear of any obstructions by any chance, and that all pins looked good?

Next step, and you may have already done this, is to verify each ram module one at a time.

Are you using two dimms or 4 dimms of ram in total?

Lastly, as my most obvious checks before going ahead, have you updated your bios, and tried a CMOS reset to make sure that it isn’t some wonky bios setting that changed and caused the ram slot to not appear?

Yes, I checked everything and tried everything. I updated my BIOS, tried CMOS and its still not working. I can try update to the last Bios patch but i think the motherboard is just not working. 

If you've checked everything and tried everything, RMA'ing is your last resort.