Sometimes if you have a sim mourn at their grave, that might help. I am sure it is purely coincidental, but they seem to show up sooner if a sim mourns. I think they come out whenever they are good and ready. In my Redhouse story, it was around 6 or 7 sim days and finally Christopher came out and then I followed him around with ambrosia, until finally I just left it alone and then he ate it.
I have taken ambrosia to the cemetary before and just left it on the ground and I was lucky enough that the sim I had left it for actually ate it. Now mind you, this was before the ghosts started coming out of their graves and heading for parts unknown. I have not tried it since the ghosts do the vanishing acts, so to speak, lol. Since I have seen the Tragic Clown now twice, it is making me want to try to bring him back. He doesn't really look any different than any other sim, mine had a top hat on and that was it. I thought he would look like an actual clown, but I guess not.