A1 and B1 memory slots don't work, while A2 and B2 do

USA B1 B2 Visit Visa - Get Early Appointments b2 slot B2 slots: B1B2 visa slot availability b200m slot As the RAM seams to work fine when put in slot B1 and B2 I guess they are ok Is it most likely a faulty motherboard or CPU that causing this?

b2b slot slot , which receives a threaded thumb