First of all, the question is already answered here ... https://discussions.apple.com/thread/8600199
Just to make it easier for you, try the following:
Open Settings > General > Accessibility > tap on 3D Touch and turn off the switch. Restart your iPhone.
After restart, turn on 3D touch again by following the above steps.
If it doesn't work, try following:
Adjust 3D Touch Sensitivity
Settings > General > Accessibility > 3D Touch > drag the slider towards left or right to adjust the sensitivity
And yes, make sure if you screen protector is not to thick. If nothing helps, Apple Care is the place you should go.