Ideas
I have had this problem myself and 100% of the time I find it is a routing issue even if the sim does not indicate it. I use bb.moveobjects on quite a lot to place items, and then I also will select the item, hold down alt and be able to freely move the item to place it just how I want it. It can take a while to narrow down because I will place things and it looks like plenty of room and should be no problem. The best example I can give is a plant I had to the side of a doorway. It was a small bathroom that I decided I wanted to add a shower to and keep the tub too. But I didn't want a shower/tub combo. After moving everything around it looked like it should be fine but I did have to use alt select to slightly move a plant that was to the right side of the door because it was touching the bathroom cabinet I placed by it and it didn't look right. When I finished the plant was just right on the edge but not in or blocking the doorway. Only I guess it was because it turned out to be the whole problem because my sim could not go into the bathroom. Also had this happen with a bookshelf that I scooted too close to the door. Again it looked fine and wasn't blocking the door but it was simply too close and the game/mysim could not navigate because of it. I am not saying that what you all are experiencing is due to this but it sure sounds the same and I remember being so frustrated because I couldn't see what the problem was. You saying that you can click on different areas and eventually get them where you want them to go really makes me feel that something is indeed happening as I described above. So if you can get them there by taking one route, look very hard at the alternative routes they might take. And remember if can be anything from a painting or wall item that juts out too far, to a ceiling light that hangs too low, or something you carefully placed that is interfering. The lights are a HUGE problem that people don't think about a lot. Anyhow, I feel your frustration and wanted to share in case my experience can help someone.