Can't make two sims sit together

[simsonata]

I can't make two sims sit on anything in my living room together. One goes to the couch or love seat and I tell the other to sit too, only for her to do the arm waving and shrug thing, with that first sim in the bubble. Why is this happening?

[Dellena]

There may be something blocking them.  Try getting them into a conversation standing up.  Then have one click on the love seat and select "Sit Together".  See if that works. 

[Joria]

I've had a similar problem.  To fix it I have my Sim walk to just in front of where I want her to sit and then say sit.  If she is across the room or around another piece of furniture I get the arm wave thing.