Could it be you have to move him in?
You do need an open spot in between your active Sims to do so.
Could you share some screenshots?
I know I'm replying in June but whatever. I've been having the same issue with my sim's husband who is also an NPC. Their relationship level is 8 and my sim lives with her brother who is a playable adult (the parents are retired and moved out).
Below is my house set up; 3 bedrooms, 4 beds, a bathroom, a kitchen and living room.
Also below is my sim and her husband along with the actions. See how the move in box is gray? It says the household is full when there's literally two people.
I planned on moving her brother's girlfriend in eventually, but until this is fixed, I guess not.
Edit: The woman with pink hair is the brother's crush
