I posted about this issue before, and the response I got was to purchase the Legendary Host moodlet thing, then throw a party with the Sim invited. I did it on three separate occasions, and it didn't work. I've also used resetSim. I didn't work. The Sim was very pregnant when she was invited over. I've read in a couple different forums that that could be a reason for this happening. However, they never gave a way to fix it. ):