Motherboard only detecting half of RAM (and slower speed)

Hi wonderful, helpful people! Major memory problem here, wonder if you can help…

I just upgraded my motherboard and CPU but kept everything else from my old rig, including the DDR4 RAM.

I initially put all 4 sticks in the new motherboard but the system wouldn’t boot - all lights and fans were on but no display. Power led was flashing quickly which apparently meant a RAM issue.

I went down to 1 stick and it booted OK, but only showing half of the RAM amount (4GB rather than 8GB) and at a slower speed (2133mhz rather than 2400).

I can get another stick in, but again it only recognises half of the total RAM, at the slower speed.

Also, only DIMMS B1 and B2 work for this - if I put anything into A1 or A2 it won’t boot at all.

Any ideas what’s going on? Is it just a duff motherboard?

Motherboard is an ASUS Prime z790-p WIFI D4.

RAM is 4x sticks of Corsair Vengeance LPX 8GB DDR4 (PC4-19200 (2400), Non-ECC Unbuffered, CAS 14-16-16-31, XMP 2.0, 1.2V )