I have 2 matching 8GB DDR3 sticks that don't seem to work. I currently have one other brand of 8GB stick in the slot and it's working fine. The motherboard manual says it can take up to 128GB. I tried using each one of the new ones alone and got the same problem. I'm fairly sure I'm inserting them correctly.
If the two modules you are using are from differing kits then you can have a mismatch due to slight Latency/Density variations. Best is to have one kit (2x8) at a frequency the CPU and MB support that are selected from the MB QVL. This way you know they have been tested and known to work.
If you want a guaranteed way to look for RAM for your motherboard, I would suggest going to your motherboard manufacturer's website > Go to their Support page > Memory
It will list all the RAM that has been tested to work with the said motherboard, how many sticks of the same RAM module were working when tested, the voltage of the RAM, etc.
This will steer you into the clear especially when you're buying high capacity RAM modules. You definitely don't want to end up with an expensive 16GB or 32GB RAM stick that doesn't even work with your system.