I have an iPhone 6s and noticed that the 3D Touch feature doesn't work on the very right side of the screen, even though the phone a detects the tap. For instance, I can be viewing my Camera Roll or Instagram and if I press try to "peek" press on the right side with my thumb just barely off the right side of the screen, then it just detects the tap but not "peek" open the photo no matter how hard I press. In Safari, I even tried placing a link in the same area on the right side and it wouldn't "peek" open the link. And it doesn't matter how high or low or link I try this on the screen. The equivalent to this on the left side is detected by bringing up the app switcher. I've tried this with 2 different 6s' and even phones in the Apple Store so I know it's not just mine. Hopefully this is just a software issue.