New AM5 PC Only Boots with One RAM Stick

Before returning anything, verify the exact slot behavior with a simple matrix: test each stick alone in A1, A2, B1, and B2, then test both in the manual-recommended pair. If both sticks work in A2 but neither works in B2, the problem is probably not the kit. If one stick fails everywhere except A2, the module itself could be faulty. Run the tests with EXPO off and no overclocking.

The 9800X3D can run on a B650 board, but compatibility and memory training can still vary between boards and individual CPUs. Make sure the BIOS update was actually completed and that the board is running its default settings rather than an old saved profile. If one channel consistently fails after CMOS resets and socket inspection, exchanging the motherboard is a reasonable first step because the symptoms resemble a defective channel or DIMM slot.

Start with the motherboard manual's recommended two-DIMM layout, which is usually A2 and B2. Leave EXPO disabled while troubleshooting, install both sticks, clear the CMOS, and give the system several minutes for DDR5 memory training. Also make sure each module is fully seated and that the CPU cooler is not tightened unevenly. If the system still only works in A2, test each stick by itself in every slot and record the results.

Since both modules work individually in A2 but the other channel does not, this points more toward a motherboard slot or memory-channel problem than a bad RAM stick. Inspect the DIMM slots and the CPU socket carefully for debris or bent pins, because the AM5 socket connects directly to the memory channels. Reseat the CPU and cooler if necessary, then retest at default settings.