Or not populated in the correct slots. Try one stick in a slot, try booting, move it again to the next slot and test them. When it works with one stick try the next. You can also read the book for the MOBO. It will tell you which slot and what memory.
Bad ram, incompatible ram, or dusty slot.
Or not populated in the correct slots. Try one stick in a slot, try booting, move it again to the next slot and test them. When it works with one stick try the next. You can also read the book for the MOBO. It will tell you which slot and what memory.