I also had this problem , was advised by the seller to make sure that the backlight was fully covered by metal LCD plate (Although what this would have to do with Force Touch I have no idea)
Ended up getting the entire phone replaced ......... but will be following this post with interest!
Ok for everyone following this thread here's an update. I received an email today from one of my suppliers saying the following:-
"iPhone 6S 3D active touch function issue is solved successfully by our factory.Our iPhone 6S screens are with sensitive and perfect 3D active touch function."
iPhone 6s 4.7 inch screens with nice 3D active touch function:
Unit price: $118.1 for AAA made by original lcd, original back light, original flex cables.
iPhone 6s plus 5.5 inch screens with nice 3D active touch function: Unit price: $175.5 for AAA made by original lcd, original back light, original flex cables.
So it looks as though 3rd partys have figured out how to keep 3D Touch / Force Touch working properly now! They said initially with the 6S that the 3D touch was causing big problems but their engineers fixed it.
I guessed (possibly incorrectly) that posting the supplier contact information here wouldn't be allowed for some reason? If that's wrong please let me know!