Try looking at your motherboard manual to see which dimm slots should be used first since putting memory in any slot could break the dual channel. Not giving your motherboard access to that ram. Or you probably used wrong memory since your memory has to be the exact same size and speed and type. Because 8gb ddr3 will not work with 8gb ddr4. 8gb ddr4 2400 MHz will also not work with 8gb ddr4 3200 MHz. And 8gb 3200 Mhz would not work with 16Gb 3200 mhz. Your ram should have the exact same specs.
