I built a PC 9 yrs ago, for a friend, with an Asus P7P55D-E mobo and 2 G-Skill 4Gb memory modules. This week, he called to say he ordered 2 more 4Gb modules and wanted me to install them. When I arrived, the PC was on, and I noticed that Windows 10 only recognized 4Gb. I powered off the PC. The first two modules were installed in slots A1 and B1. I installed the two new identical 4Gb modules in slots A2 and B2. Upon rebooting, the Bios only recognizes 8Gb. Windows Task Mgr says 8Gb in 2 slots out of 4 slots. I looked at the Asus mobo manual and it says I can install a single module in either slot A1 or B1. Using one module, I installed it in slot A1. The PC would not boot. I Installed it in Slot B1 and it booted fine. I am not sure if I need to press the Mem OK button and when. It says it will not work in the Windows OS so I assume you should only use it when in the Bios Setup Utility.