Try removing the bush from around the house to see if that gets rid of the issue. It does sound like there's something not right with the house.
Is the foundation closed all around ?
I removed the bush, but they still won't do anything.
Now there are just the foundations, and the stairs (of course there's still the house built on the foundations, but I don't think there's anything wrong with it, I built it the same way I built all the others and they all work fine).
I found someone who had the same problem, and they resolved it with an update, but I updated my game lately :S
What is your base game version right now ?
To find out, open your launcher and hover over the little base game icon in the lower left part of the launcher. It should say 1.50.
More Details can be found here: http://sims3.crinrict.com/en/2011/01/tutorial-how-to-find-your-game-version.html
For Macs: http://bluebellflora.wordpress.com/the-sims-3-on-a-mac/checking-which-patch-version-youre-running/
It says it's 1.50
I'm having the same exact problem right now. My sim wont do anything except use stairs and the bathroom. Someone please help ☹️
