1) Are these memory sticks designed to run at the same voltage?
2) Do they share speeds and timings?
CPU:
Motherboard:
Old RAM:
New RAM:
My suggestion would be to take out all the memory and try just the new ram to make sure that it works at all. If so, you likely have memory that is incompatible. At that point you can try to install the old ram, and it might have a different result. It should default to 2133 with loose timings. Then it would be a matter of manually settings timings and speeds that will work with both.