Okay I finally figured out what my problem with this was, and I thought it might help some of you. I could find the yellow line and everything but not for the right room, and then I realised that it wasn't technically a room, it was just surrounded by rooms (second floor). So when I though I selected the room I wanted to add a ceiling to, I was actually clicking the room below it. I tried to build walls to make the room AN ACTUAL ROOM. But it just wasn't working and I finally just had to give up and pull another room on that floor until it was one gigantic room, which is really frustrating and not working with my plans for this floor but at least I have a ceiling right?