Is there more than one door on the lot?
I built a house for an NPC family and when I moved them in, I found that I couldn't knock on the front door. I looked around and apparently the game thought that the back garden gate was the front door and I could knock on that.
So I went back into build mode and deleted the back gate so that the only door was the front door. That solved it for me.
The other option is to choose to play the NPC family for bit. While you are controlling them you can click on the door and choose the "set as front door" option. I don't know if that sticks as the door you chose if you make them unplayable again.
Tagging @BryonyRae to see if she has further info on this
I build a house underground , its just a stairway lead up to the surface . the door is at the bottom of the steps.. when the new neighbor came over to say hello they didn't knock on the door they just stood there waiting on me... I do have a pic though
