3D Touch is a new feature found only on the iPhone 6s and 6s Plus. If you own one of these devices and the feature still isn't working here are some troubleshooting tips.
Make sure 3D Touch is enabled :
You can click here to learn how to enable/disable 3D Touch.
Adjust 3D Touch sensitivity:
3D Touch works by recognizing the pressure of your touch. You can follow these instructions to learn how to adjust the pressure for 3D Touch transitions.
Using 3D Touch with third-party apps:
Third-party apps have to support 3D Touch for Quick Actions and Peek and Pop features to work. You can check the App Store description of the app you're using to see if it supports 3D Touch. Remember, even if an app does support 3D Touch it may not support all the features.