I had this exact issue with my FX-8350. I didn't use it for a few years, and when I socketed it a couple of weeks ago I was only able to see 8GB / 16GB RAM (4x4 config). I got a new FX motherboard and CPU just to see what the issue was, and it was the CPU. No issue with either motherboard on the new CPU.
So I do not know the cause of this, but from what you are saying and my personal experience, it sounds as though your particular CPU may not be able to see 2 of the DIMM slots. I don't know if this is a common issue with FX CPUs. As a result I also don't know how to fix it, aside from a new CPU. FX-6300 isn't too expensive on Aliexpress, if you are able to wait.