Memory upgrade

I installed two 8 GB chips in my iMac (Retina 5K, 27-inch, Late 2015) which I purchased from Adamanta. The computer however only registers them as 4 GB each giving me a total of 24 instead of 32 GB. I contacted the vendor and they sent me out two new 8 GB chips but after installation the computer still indicates that each chip is only 4 GB. The computer should be expandable to 32 GB RAM. Any suggestions on what the issue(s) maybe and how to fix? Thanks in advance for any assistance.