I've had a 100% success rate with that one. That is to say, in all the years I've been playing TS3 I've summoned a grand total of one mummy and she showed up right away. Just a few weeks ago, in fact. Guess that's not a very statistically relevant sample set though.
One thing we know that goofs this up is if the active sim doing the summoning has too many of the ancient canopic jars -- that is, more than one complete set or a complete set plus the beginnings of more sets. It's always been said that the "extra" jars need to be shifted to a different sim or stored elsewhere for this to work. I suppose other common issues would be that maybe there isn't enough room around the sarcophagus for the sequence to complete or that your sim's household is full?