Mixing RAM modules is not guaranteed to work. This is why manufacturers sell RAM in packs. Because technically, the packs are the only ones guaranteed to work together.
The fact that they might be the same size / make / model is irrelevant.
Obviously I'm assuming that you've also plugged the RAM into the slots as per the motherboard manual? You can try each stick individually, and each slot on the motherboard to confirm if there is a physical hardware problem.