Why is MacBook Pro 15" only recognizing 8GB of 16GB RAM Upgrade?

Yesterday, I went to the local MicroCenter store to purchase a memory upgrade kit for my MacBook Pro 15" unibody mid-2012 edition. I have installed 2 8GB RAM memory modules, but the MacBook is only recognizing 1 of them. Can anyone help determine why only 1 of the 2 modules is being recognized? I am out of luck? Is it possible that I didn't securely seat 1 of the 2 modules in the computer?

After looking at other posts, I can confirm the following about my MacBook Pro:

Model Identifier: MacBookPro9,1

Boot ROM Version: MBP91.00D3.B0E

SMC Version: 2.1f175

So, I have confirmed that I am running the latest firmware, etc. per the question asked and answered here:

Install fully functional 16gb ram on my MacBook model 7,1?

I installed Crucial brand memory manufactured by Micron and designed for the Mac. The kit includes 2 8GB DDR3L-1600 SODIMM 1.35V CL11 memory modules. Crucial Web site actually shows that this should be compatible as well though the "model number" of the RAM they are currently selling differs from what I bought.

My Crucial Memory model #: CT2C8G3S160BM

The compatible model # on Crucial Web site: CT3327432