You may have to reset the cmos or bios to get the system to boot with the other stick, If you want to try it here are the steps.
1) Power down the system
2) Unplug the system from the wall or pull the power from the back of the Power supply
3) IMPORTANT! Hold the power button down while its unplugged from the wall to drain the power, Fans could spin for a brief moment or lights might flash
4) Open the side panel and locate the button cell battery, some call it a watch battery, sometimes its in a bad spot like under the graphics card
5) Pop the battery out, and wait about 60 seconds
6) Reinstall the button cell battery with the writing facing out and reinstall the graphics card if you have to
7) Make sure the Graphics card power is plugged in and plug in the system and see if it boots with the new stick of ram
If you do this, the system may take a while to boot and even restart several times to reconfigure the bios for your hardware, Wait a bit and see if anything is displayed on the screen, If it fails to boot, that means that stick of ram wont work with your already existing sticks, I'd return it at the point.
It happens even if they are the same brand and model even.
I had 2 kits of Corsair Vengeance Pro 3000mhz ram, they came in a kit with 2 8Gb sticks in each kit, Numbers looks and everything was the same, but the revision numbers were different between both kits. they would not work at all together, but they would work fine by them selfs.
Unfortunately if you don't want to take the battery out, the only option would be to take the stick back for a refund, and get your self a quad channel kit, sometimes you have ask an employee as they are sometimes not on the self's or display (at least thats what Micro Center does). And try that.
You can always return the kit if it don't help your performance, just make sure you install them stagerd or in the same colored slots exactly like that MSI board in the Pic I linked for Quad channel to work. If that dosn't help, A GPU upgrade to the 1080ti might not improve your game performance much in PUBG due to the low clocked CPU, Overclock it some and the game will start to perform better.
You can also reinstall the video drivers to see if performance improves, Sometimes the drivers go crazy after a few updates and it needs to be reinstalled for the best performance.
You can see if your Titan is limiting you by downloading something like MSI afterburner, and watch the graph as you play PUBG, Watch the GPU usage, if the graphics card usage is at 100% then a graphics card upgrade would help you as that means you are GPU bottleneck, which isn't bad if you are getting the desired FPS, but if its at anything but 99% and FPS is not where you want it, than that means the CPU is holding the card back or its a poorly optimized game which is typically CPU heavy.
Because you have 6 core i7 dosn't always mean you will get the best performance, Clock speeds do matter to an extent. Intel often lowers the clock speeds of their high core count chips which can negatively impact performance in some games compare to lets say a higher clocked 4 core i7. But with a few tweaks in the bios you can have it perform nearly as well if not better in some cases than a stock i7 7700k.
Hopefully I didn't confuse you lol.
Post back if you need more help!
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