If you had DDR 3 modules in at a slower speed.
And the new ones are rated at a higher speed.
Factory default the bios on the board.
The old values are still retained causing a conflict.
Also check that each memory slot can take the capacity or density of the ram module.
If its more Gb per stick.
Some boards have a limit on how much memory it can address on each dimm slot. This such a case when the board has only two dimm slots vs four.
It cannot handle a population density of memory on a stick due to the memory controller of the Mobo so you check the max size of memory that can be fitted to a slot.
Make sure the ram stick are fully seated level you may have to apply some force till they sit right in the dimm slots, both the...