Pretty much what @TreyNutz said. I would add that it can often help to "follow" your sim (at least keep them as active, if the camera isn't watching them) and keep reassigning the task, if necessary. Sometimes the other sim is in the middle of a task, or needs to leave the house, or has to wait for another sim to stop talking. But if I'm really having trouble, I just find the star tag on the map that shows the location of the other sim, send my own sim there, and keep spamming the interaction until it works.