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What you describe can be caused by one or more bent pins on the CPU, it is common for a bent pin/s to result in 1 or more memory channel being lost. Have you checked your CPU pins and CPU/RAM socket for debris/dust or any other obvious issues/defects.
-You can test each stick of memory separately in the working slot/s to make sure it is not the ram stick/kit that is bad. HCI memtest is free and easy to use. Directions can be found on the Ryzen memory stability thread at Overclockers.net.
-Have you tried the RAM in B1B2 slots? If it works and reads both channels I do not think there is any penalty to performance.
- If it is bent pins there are guides online on how to straighten, I have done this multiple times and used a razor-blade to straighten the bent pins.
- If no pins are bent and the RAM kit can pass memtest at 90% coverage at 100% (IMHO) you should contact your motherboard manufacture for troubleshooting or replacement.