First, you should probably verify the model of your phone and make sure you've got the right part. I'm pretty sure a fully assembled iPhone 6s display should have three connectors, not four.
Next, compare the new display with the old display. Are they identical? Are the connectors physically different? Sometimes the display ribbon cables get folded in the wrong order so they won't line up properly, and in that case it's a simple matter of carefully reorganizing the cables so they line up with their sockets.
Let us know what you find and maybe we can help further. Good luck!
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If it has 4 connectors as you say you got the iPhone 6 screen replacement. The 6s only has 3 connectors.
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I had the same problem and it has nothing to do with after market parts. When you switched the camera assembly to the new case, you put that cable under the other cables, reversing the order. Remove the shield, and place the camera cable on top of the 2 larger board cables and all will fit properly
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