If you're only using B1 & B2, you are losing memory bandwidth due to only using a single bank. Some tools may report that as "dual channel" with DDR5 because there's 2 channels per bank, but those tools would report "quad channel" if you were using both A & B banks (DDR4 was 1 64 bit channel per DIMM, DDR5 is 2 32 bit channels per DIMM). Possible causes are CPU/cooler mounting pressure, CPU socket damage, or a board problem. Does it work with XMP/EXPO disabled (i.e. is this an overclocking problem or are the A sockets completely dead)?
If you've not already done so, carefully inspecting the pins in the CPU socket for damage / contamination / etc is one of the usual next steps, followed by carefully remounting the CPU. Those pins are very delicate, so don't poke at them, removing any contamination / debris from the socket must be done with great care. Also inspect the contact pads on the base of the CPU for any anomalies. Don't excessively torque down the cooler mount, that can cause the board/socket to bend and prevent all pins making good contact.
