ASRock motherboard not recognizing all four RAM slots

I recently bought 2 packs of 16GB G.SKILL Ripjaws X Series RAM from a person online. According to the seller, the RAM was going to be used in a build but the build was put off and the RAM was never used. Here is the link to the item details.
When I received the RAM and installed it (before you ask I made sure the RAM was seated correctly), my ASRock Z68 Pro3 Gen3 motherboard only recognized 16GB dual channel and not the whole 32GB. The RAM has run at 1600 MHz and is now running stable at 2133 MHz. The mobo supports up to 32GB.
I don't believe the RAM is bad, so excluding faulty memory, how would I get the mobo to recognize all four slots?
ETA: I believe the issues may lie in the configuration of the RAM. It may not be recognizing all the RAM because of dual channel. I'm not sure but if I could get someone's clarification on the matter I would really appreciate it.

Disregard the post. Seller advertised the RAM as 4x8GB sticks but they were actually 4x4GB sticks.