Try and watch the behaviour of all the fish over an extended period of time to identify the culprit(s).
If more than one culprit, add a few more hiding areas away from where the culprits usually congregate for the affected fish to escape to and feel secure.
These cichlids are notorious for being aggressive and often for no good reason, they are just wired that way. If only one fish is being exceptionally aggressive, the choices are quite limited...get it to another aquarium/rehome it or in some cases and if you have a big enough aquarium a divider can work. But since they are so naturally territorially aggressive, its often very hard to stop the behaviour whilst the offending fish are still in the aquarium and the only option is to remove either the bully/bullies or the victim permanently, once the bullies sense a weaker fish, they will forever go after it, stress it and potentially physically attack til it dies.