Whenever you ad a new cichlid to a tank with already established cichlids you are taking a chance that the most dominant cichlid (in your case the festae) in the tank will either outwardly kill, or kill by intimidation the new cichlid.
Intimidation (high stress) can cause the immune system of the new fish to quickly break down.
The resident cichlids have invested heavily in their territory, the new one is at a disadvantage. This is why adding cichlids piecemeal to a tank is always a bit of a gamble, and why it is best to ad all fish at once. Once territories are established it is very hard to change things.