Hershey does not want to get ganged up on by the others.

Fifi was an only cat when we got her as a companion for Lamoni. She was so scared that when she did something bad I would punish her by showing her a mirror. Oh boy did she ever growl at that other cat.

One day she charged the mirror knocking it flat. She sniffed around the edges and then looked down at the "other cat". You could almost see the "light" come on over her head. She realized that it was not another cat after all. But...

.. as she walked away Lamoni, our boy cat, fell over on his side, rolled on hes back, kicked his paws up, wagged his tail, and started meowling at the ceiling. I swear that he was laughing his head off at the silly new girl.

Let them sniff each other out in a controlled environment. Be protective of Hershey and don't let them gang up. Try bringing just one of the flock into the room for an introduction.